CLN: Remove convert_objects
- [x] xref #6581
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26612?src=pr&el=h1) Report
> Merging [#26612](https://codecov.io/gh/pandas-dev/pandas/pull/26612?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/437efa6e974e506c7cc5f142d5186bf6a7f5ce13?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26612/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26612?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26612      +/-   ##
==========================================
- Coverage   91.85%   91.83%   -0.03%     
==========================================
  Files         174      174              
  Lines       50722    50718       -4     
==========================================
- Hits        46593    46579      -14     
- Misses       4129     4139      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <ø> (-0.02%)` | :arrow_down: |
| #single | `41.78% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26612?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26612/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.61% <ø> (-0.02%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26612/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/26612/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `90.54% <0%> (-1%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26612/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26612?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/26612?src=pr&el=footer). Last update [437efa6...04eda79](https://codecov.io/gh/pandas-dev/pandas/pull/26612?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=12188

looks like a doc-string test still has this, otherwise lgtm. thanks @mroeschke 