PERF: remove useless overrides
- [x] closes #21522
- [x] tests passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Asv run:

```
       before           after         ratio
     [9e982e18]       [705f0e3b]
-         243±7ms          186±4ms     0.77  multiindex_object.GetLoc.time_large_get_loc_warm
-       220±0.9ms          151±3ms     0.69  multiindex_object.GetLoc.time_large_get_loc
-         173±1ms          101±2ms     0.59  multiindex_object.Integer.time_get_indexer

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```

Notice that as of now the same _cannot_ be done for ``.is_monotonic_increasing`` and friends, because the sortedness of the ``MultiIndex`` corresponds to the sortedness of the underlying int index only if levels are themselves sorted.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21523?src=pr&el=h1) Report
> Merging [#21523](https://codecov.io/gh/pandas-dev/pandas/pull/21523?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9e982e18678c47c83ce2bb5932f844e2d9ee21f3?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/21523/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21523?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21523      +/-   ##
==========================================
- Coverage   91.92%   91.92%   -0.01%     
==========================================
  Files         153      153              
  Lines       49587    49583       -4     
==========================================
- Hits        45583    45579       -4     
  Misses       4004     4004
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.83% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21523?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/21523/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `94.97% <ø> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21523?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/21523?src=pr&el=footer). Last update [9e982e1...131a7c3](https://codecov.io/gh/pandas-dev/pandas/pull/21523?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!