Fix c build warnings #17634; remove several unused funcs

[jbrockmendel]

Removes several unused functions, most importantly floatify which util attempts to import from numpy_helper but which does not exist there.

While putting this together I noticed in parsers.pyx a couple occurrences ofprint >> sys.stderr, self.parser.warn_msg. I guess this is still valid cython, but should probably be updated anyway.

• closes BLD: parser c-build warnings #17634
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #17932 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17932      +/-   ##
==========================================
- Coverage   91.23%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       50113    50113              
==========================================
- Hits        45723    45714       -9     
- Misses       4390     4399       +9

Flag	Coverage Δ
#multiple	89.03% <ø> (ø)	⬆️
#single	40.31% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.75% <0%> (-0.1%)	⬇️

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[jreback]

thanks!