MAINT: Drop the get_offset_name method
Deprecated since 0.18.0

xref #11834 ( :-1: for no test being added for its deprecation back then! :smile:)

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=h1) Report
> Merging [#16863](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9c44f9b2cad863bde17c7dd061d5b5b5ccbada21?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16863/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16863      +/-   ##
==========================================
- Coverage   90.98%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49300    49296       -4     
==========================================
- Hits        44857    44847      -10     
- Misses       4443     4449       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.18% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `96.62% <ø> (+0.53%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16863?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/16863?src=pr&el=footer). Last update [9c44f9b...8efff3b](https://codecov.io/gh/pandas-dev/pandas/pull/16863?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!