DOC: Remove Series.sortlevel from api.rst
https://travis-ci.org/pandas-dev/pandas/jobs/447288079#L1918

Was dropped in #15099 but accidentally added back in #18202.

Follow-up to #23375.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23395?src=pr&el=h1) Report
> Merging [#23395](https://codecov.io/gh/pandas-dev/pandas/pull/23395?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62a15fa4071d0711b2b97771b981d527c1c4af10?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23395/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23395?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23395   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files         166      166           
  Lines       51224    51224           
=======================================
  Hits        47212    47212           
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23395?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/23395?src=pr&el=footer). Last update [62a15fa...cf7e71f](https://codecov.io/gh/pandas-dev/pandas/pull/23395?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://travis-ci.org/pandas-dev/pandas/jobs/447320891#L1920

No more warning! 🎉  thanks!