DOC: remove Panel4D from the API docs #15579

[yinleon]

Hi this is my first PR to Pandas
I'm pretty sure this is what @jorisvandenbossche was getting at on ticket #15579.

I noticed that tests for panel4D also still exist, is that something that should remain?

Thanks!

[jorisvandenbossche]

@yinleon Perfect, thanks!

The tests should remain, until we actually fully remove the functionality.

[codecov-io]

Codecov Report

Merging #15598 into master will decrease coverage by -0.03%.
The diff coverage is n/a.

@@            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	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.78% <0%> (-0.35%)	❌
pandas/core/common.py	91.02% <0%> (-0.34%)	❌
pandas/core/frame.py	97.83% <0%> (-0.1%)	❌
pandas/tseries/index.py	95.42% <0%> (+0.09%)	✅

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