DOC: remove Panel4D from the API docs #15579
Hi this is my first PR to Pandas
I'm pretty sure this is what @jorisvandenbossche was getting at on ticket <a href='https://github.com/pandas-dev/pandas/issues/15579'>#15579<a/>.

I noticed that <a href='https://github.com/pandas-dev/pandas/blob/be4a63fe791e27c2f8a9ae4f3a419ccc255c1b5b/pandas/tests/test_expressions.py#L206'>tests</a> for panel4D also still exist, is that something that should remain?

Thanks!

@yinleon Perfect, thanks!

The tests should remain, until we actually fully remove the functionality. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15598?src=pr&el=h1) Report
> Merging [#15598](https://codecov.io/gh/pandas-dev/pandas/pull/15598?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7740231bbc0b92db55479cd02400b37c9470a4d2?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15598      +/-   ##
==========================================
- Coverage   91.06%   91.03%   -0.03%     
==========================================
  Files         137      137              
  Lines       49307    49307              
==========================================
- Hits        44899    44888      -11     
- Misses       4408     4419      +11
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15598?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/7740231bbc0b92db55479cd02400b37c9470a4d2...86dc3976af0fb188be60cd77eb0ad3bfe41b0844?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/7740231bbc0b92db55479cd02400b37c9470a4d2...86dc3976af0fb188be60cd77eb0ad3bfe41b0844?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.78% <0%> (-0.35%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/7740231bbc0b92db55479cd02400b37c9470a4d2...86dc3976af0fb188be60cd77eb0ad3bfe41b0844?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <0%> (-0.34%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/7740231bbc0b92db55479cd02400b37c9470a4d2...86dc3976af0fb188be60cd77eb0ad3bfe41b0844?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.83% <0%> (-0.1%)` | :x: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/compare/7740231bbc0b92db55479cd02400b37c9470a4d2...86dc3976af0fb188be60cd77eb0ad3bfe41b0844?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.42% <0%> (+0.09%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15598?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15598?src=pr&el=footer). Last update [7740231...86dc397](https://codecov.io/gh/pandas-dev/pandas/compare/7740231bbc0b92db55479cd02400b37c9470a4d2...86dc3976af0fb188be60cd77eb0ad3bfe41b0844?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).