CLN: simplify combine_first

[jreback]

No description provided.

[codecov]

Codecov Report

Merging #20972 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #20972      +/-   ##
==========================================
+ Coverage   91.81%   91.81%   +<.01%     
==========================================
  Files         153      153              
  Lines       49481    49474       -7     
==========================================
- Hits        45430    45427       -3     
+ Misses       4051     4047       -4

Flag	Coverage Δ
#multiple	90.21% <100%> (ø)	⬆️
#single	41.85% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.22% <100%> (+0.09%)	⬆️
pandas/util/testing.py	84.59% <0%> (+0.2%)	⬆️

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