Remove unused calendar options from period_helper

[jbrockmendel]

period_helper supports GREGORIAN_CALENDAR and JULIAN_CALENDAR, but only Gregorian is ever used. This removes that argument, which will allow for the following in follow-ups:

• Remove a bunch of error checking and propogation boilerplate.
• Get a a bunch of functions from datetime/np_datetime.h instead of re-implementing them in period_helper.
• Do the same removals to simplify WIP: implement period_helper in cython #19498

[jbrockmendel]

Travis timeout looks unrelated.

[codecov]

Codecov Report

Merging #19534 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19534      +/-   ##
==========================================
- Coverage   91.62%   91.59%   -0.03%     
==========================================
  Files         150      150              
  Lines       48714    48714              
==========================================
- Hits        44633    44619      -14     
- Misses       4081     4095      +14

Flag	Coverage Δ
#multiple	89.96% <ø> (-0.03%)	⬇️
#single	41.75% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️
pandas/util/testing.py	83.64% <0%> (-0.21%)	⬇️

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[jreback]

thanks!