Remove unused calendar options from period_helper
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 #19498 