CLN: Remove the Panel4D and PanelND classes
Deprecated back in v0.19.0

xref #13147
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19059?src=pr&el=h1) Report
> Merging [#19059](https://codecov.io/gh/pandas-dev/pandas/pull/19059?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6552718d2a908a57b88354fe916609631793131e?src=pr&el=desc) will **decrease** coverage by `0.06%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19059/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19059?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19059      +/-   ##
==========================================
- Coverage   91.56%    91.5%   -0.07%     
==========================================
  Files         150      148       -2     
  Lines       48942    48852      -90     
==========================================
- Hits        44816    44701     -115     
- Misses       4126     4151      +25
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <100%> (-0.06%)` | :arrow_down: |
| #single | `41.64% <66.66%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19059?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `96.84% <ø> (-0.01%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.45% <ø> (-0.3%)` | :arrow_down: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.9% <100%> (-0.05%)` | :arrow_down: |
| [pandas/core/dtypes/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2dlbmVyaWMucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.45% <100%> (-0.4%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `92.62% <0%> (-0.19%)` | :arrow_down: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/19059/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19059?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/19059?src=pr&el=footer). Last update [6552718...b2b478f](https://codecov.io/gh/pandas-dev/pandas/pull/19059?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 love it! thanks! (and am NOT sad that you blew away a lot of my original code), lol.
   @jreback : If you have any emotional attachment to that code, I'd be more than happy to create a separate branch where it lives on :smile:  hahah
i still prob have the original branch 
i don’t usually delete them (so i have thousands :) > i still prob have the original branch
i don’t usually delete them (so i have thousands :)

Awesome, glad we resolved that (non-)issue :)