CLN: remove deprecated infer_dst keyword

[jorisvandenbossche]

This has come up a few times during the docstring improvements: we still had the infer_dst deprecations left over (deprecated in 0.15)

[codecov]

Codecov Report

Merging #20490 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #20490      +/-   ##
==========================================
- Coverage   91.85%   91.85%   -0.01%     
==========================================
  Files         152      152              
  Lines       49235    49232       -3     
==========================================
- Hits        45224    45221       -3     
  Misses       4011     4011

Flag	Coverage Δ
#multiple	90.24% <100%> (-0.01%)	⬇️
#single	41.88% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/datetimes.py	95.73% <ø> (-0.01%)	⬇️
pandas/core/generic.py	95.85% <ø> (-0.01%)	⬇️
pandas/core/indexes/period.py	92.61% <100%> (ø)	⬆️

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[jreback]

thanks @jorisvandenbossche