API/VIS: remove misc plotting methods from plot accessor (revert #23811)
To be clear: I am opening this for discussion, but directly as a PR instead of an issue so it is easier to merge it in case we agree.

https://github.com/pandas-dev/pandas/pull/23811 added the misc plotting methods (like 'andrew_curves', 'parallel_coordinates', .. see http://pandas.pydata.org/pandas-docs/stable/visualization.html#plotting-tools) to the `DataFrame/Series.plot` accessor. 

But personally, I think we really shouldn't expose those methods too much (we should rather be deprecating them, but we only kept them for historical reasons). 

cc @jreback @mroeschke 
I tend to agree as this should probably live in seaborn(?). If we all agree may as well go ahead with the deprecation as well in this PR, unless this PR is intended to make it in before 0.24 >  unless this PR is intended to make it in before 0.24

Yeah, if there is agreement, that is my intention. (as this is new on master, not yet released, so now we can still remove it without deprecating) seems fine by me. yeah this does reduce surface area. Maybe should completely deprecate these though? (that is more work) +1 to not adding to `.plot`.

I'm fine with just keeping them, since they don't currently have much maintenance burden. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=h1) Report
> Merging [#24912](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5761e359b65631db491349a65fc21d0da51dcc0f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24912/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24912      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52404    52389      -15     
==========================================
- Hits        48413    48397      -16     
- Misses       3991     3992       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/24912/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.58% <ø> (-0.18%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24912/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24912?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/24912?src=pr&el=footer). Last update [5761e35...0b6e4c7](https://codecov.io/gh/pandas-dev/pandas/pull/24912?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/24912?src=pr&el=h1) Report
> Merging [#24912](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5761e359b65631db491349a65fc21d0da51dcc0f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24912/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24912      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52404    52389      -15     
==========================================
- Hits        48413    48397      -16     
- Misses       3991     3992       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/24912/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.58% <ø> (-0.18%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24912/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24912?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/24912?src=pr&el=footer). Last update [5761e35...0b6e4c7](https://codecov.io/gh/pandas-dev/pandas/pull/24912?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > I'm fine with just keeping them, since they don't currently have much maintenance burden.

I also don't feel the urge to remove them since they are not top-level exposed, but would also not object deprecating them. In any case, for another issue/PR after the release. BTW, we could maybe discuss to keep scatter matrix as one exception