PERF: remove useless overrides

[toobaz]

• closes MultiIndex.is_unique is unbearably slow #21522
• tests passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• 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]

Codecov Report

Merging #21523 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            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%)	⬇️
#single	41.83% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/multi.py	94.97% <ø> (-0.02%)	⬇️

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[jreback]

thanks!