CLN: remove Panel-specific parts of core.generic
xref #25632

removed small amount of code related to self.ndim > 2

also a few changes to make the NotImplemented errors for self.ndim>2 more consistent and added test for these. (might be worth having a decorator for these)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=h1) Report
> Merging [#26738](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c6fdb6b38a44bb05a2f6c1224fcc2e30e40981b4?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26738/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26738      +/-   ##
==========================================
+ Coverage   91.78%   91.79%   +<.01%     
==========================================
  Files         174      174              
  Lines       50703    50695       -8     
==========================================
- Hits        46538    46533       -5     
+ Misses       4165     4162       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <100%> (+0.01%)` | :arrow_up: |
| #single | `41.81% <0%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26738/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.94% <100%> (+0.3%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26738/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26738/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26738?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/26738?src=pr&el=footer). Last update [c6fdb6b...ff2747d](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=h1) Report
> Merging [#26738](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3937fbc5fa7d4228c65e863a24d69fa4b7cf4ec7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26738/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26738      +/-   ##
==========================================
+ Coverage   91.78%   91.78%   +<.01%     
==========================================
  Files         174      174              
  Lines       50703    50683      -20     
==========================================
- Hits        46538    46521      -17     
+ Misses       4165     4162       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <100%> (+0.01%)` | :arrow_up: |
| #single | `41.82% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26738/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.91% <100%> (+0.27%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26738/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26738/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26738?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/26738?src=pr&el=footer). Last update [3937fbc...a2cedd7](https://codecov.io/gh/pandas-dev/pandas/pull/26738?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks, @simonjayhawkins!