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!
