CLN: remove obsolete code in Categorical.fillna

[cfgong]

closes #19787

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry
  Deleted code and tested block of code associated with NaN that should be unreachable.

[TomAugspurger]

Thanks. Looks like some test cache files got in your commits.

Could you

1. Delete those files: rm -rf .pytest_cache
2. Remove them from the next commit (I think git rm .pytests_cache
3. Add .pytest_cache to the .gitignore file at the root of the pandas directory?

And then push those changes.

[codecov]

Codecov Report

Merging #19880 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19880      +/-   ##
==========================================
+ Coverage   91.78%   91.78%   +<.01%     
==========================================
  Files         152      152              
  Lines       49196    49191       -5     
==========================================
- Hits        45153    45151       -2     
+ Misses       4043     4040       -3

Flag	Coverage Δ
#multiple	90.17% <ø> (ø)	⬆️
#single	41.82% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/arrays/categorical.py	95.47% <ø> (+0.36%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 38afa93...a1df599. Read the comment docs.

[jreback]

thanks!