Remove unused files from src/klib

[jbrockmendel]

De-couple ujson from numpy_helper

[codecov]

Codecov Report

Merging #19415 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19415      +/-   ##
==========================================
+ Coverage   91.62%   91.65%   +0.02%     
==========================================
  Files         150      150              
  Lines       48729    48729              
==========================================
+ Hits        44649    44661      +12     
+ Misses       4080     4068      -12

Flag	Coverage Δ
#multiple	90.02% <ø> (+0.02%)	⬆️
#single	41.75% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[jreback]

thanks!