CLN: simplify CategoricalIndex internals

[topper-123]

Simplify method signatures of some internal methods of CategoricalIndex.

[pep8speaks]

Hello @topper-123! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/core/indexes/category.py !

[topper-123]

The travis error is unrelated:

curl: (56) SSL read: error:00000000:lib(0):func(0):reason(0), errno 104
Unable to download 3.5 archive. The archive may not exist. Please consider a different version.

[jreback]

i restarted to make sure this passes. ping on green.

[codecov]

Codecov Report

Merging #24343 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24343      +/-   ##
==========================================
+ Coverage   92.28%   92.34%   +0.06%     
==========================================
  Files         162      162              
  Lines       51835    52217     +382     
==========================================
+ Hits        47834    48218     +384     
+ Misses       4001     3999       -2

Flag	Coverage Δ
#multiple	90.75% <100%> (+0.07%)	⬆️
#single	43.16% <90%> (+0.16%)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexes/category.py	98.65% <100%> (+0.75%)	⬆️
pandas/util/testing.py	87.58% <0%> (+0.09%)	⬆️
pandas/core/arrays/datetimes.py	98.37% <0%> (+0.13%)	⬆️
pandas/core/resample.py	96.75% <0%> (+0.16%)	⬆️
pandas/tseries/offsets.py	97.24% <0%> (+0.4%)	⬆️

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[jreback]

thanks @topper-123