remove daytime attr, move getstate and setstate to base class

[jbrockmendel]

If/when #18224 is revived to make _BaseOffset a cdef class, we'll need to move __getstate__ and __setstate__ into the base class (and then make some changes). This moves the two methods pre-emptively so that we'll have a smaller diff for the next steps.

Also removes an attr self.daytime that is not used anywhere else.

[jreback]

lgtm. ping on green.

[codecov]

Codecov Report

Merging #21533 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #21533      +/-   ##
==========================================
- Coverage   91.92%   91.92%   -0.01%     
==========================================
  Files         153      153              
  Lines       49594    49570      -24     
==========================================
- Hits        45590    45566      -24     
  Misses       4004     4004

Flag	Coverage Δ
#multiple	90.32% <ø> (-0.01%)	⬇️
#single	41.81% <ø> (-0.02%)	⬇️

Impacted Files	Coverage Δ
pandas/tseries/offsets.py	97.19% <ø> (-0.06%)	⬇️

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[jreback]

thanks!