CLN/DEPR: remove pd.ordered_merge
- [ x] xref #13358
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ x] whatsnew entry

``pd.ordered_merge`` was deprecated in #13358 (pandas v.0.19). This PR removes it from the code base.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=h1) Report
> Merging [#18459](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5e670653e50dcbbafc0ba004b16328f49925f041?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18459/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18459      +/-   ##
==========================================
- Coverage   91.35%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49695    49691       -4     
==========================================
- Hits        45401    45388      -13     
- Misses       4294     4303       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <100%> (-0.01%)` | :arrow_down: |
| #single | `39.66% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9hcGkucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <100%> (-0.04%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=footer). Last update [5e67065...e1b862a](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=h1) Report
> Merging [#18459](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5e670653e50dcbbafc0ba004b16328f49925f041?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18459/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18459      +/-   ##
==========================================
- Coverage   91.35%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49695    49691       -4     
==========================================
- Hits        45401    45388      -13     
- Misses       4294     4303       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <100%> (-0.01%)` | :arrow_down: |
| #single | `39.66% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9hcGkucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <100%> (-0.04%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=footer). Last update [5e67065...e1b862a](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=h1) Report
> Merging [#18459](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aec33479b923a05ead1ca35335f00aba87e4145e?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18459/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18459      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49704    49700       -4     
==========================================
- Hits        45411    45398      -13     
- Misses       4293     4302       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <100%> (-0.01%)` | :arrow_down: |
| #single | `39.66% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <ø> (-0.04%)` | :arrow_down: |
| [pandas/core/reshape/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9hcGkucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=footer). Last update [aec3347...0e3f539](https://codecov.io/gh/pandas-dev/pandas/pull/18459?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @topper-123 