CLN: Drop .sortlevel method
Title is self-explanatory.

xref #15099.
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/gfyoung/pandas/blob/389e4f8d7c38f5dbe54dd1fbc4ebb11bb4002172/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/core/series.py`](https://github.com/gfyoung/pandas/blob/389e4f8d7c38f5dbe54dd1fbc4ebb11bb4002172/pandas/core/series.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_sorting.py`](https://github.com/gfyoung/pandas/blob/389e4f8d7c38f5dbe54dd1fbc4ebb11bb4002172/pandas/tests/frame/test_sorting.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_sorting.py`](https://github.com/gfyoung/pandas/blob/389e4f8d7c38f5dbe54dd1fbc4ebb11bb4002172/pandas/tests/series/test_sorting.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23375?src=pr&el=h1) Report
> Merging [#23375](https://codecov.io/gh/pandas-dev/pandas/pull/23375?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f662c5f2a7b392ee96a43f055073c8523f8f0339?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/23375/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23375?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23375      +/-   ##
==========================================
- Coverage   92.22%   92.22%   -0.01%     
==========================================
  Files         169      169              
  Lines       51261    51255       -6     
==========================================
- Hits        47277    47271       -6     
  Misses       3984     3984
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23375?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23375/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.03% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/23375/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.89% <ø> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23375?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/23375?src=pr&el=footer). Last update [f662c5f...389e4f8](https://codecov.io/gh/pandas-dev/pandas/pull/23375?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!