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 
Travis timeout looks unrelated. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=h1) Report
> Merging [#19534](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/de39a1572fcf82071f3c0b5f22be1611222bdf41?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19534/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=tree)

```diff
@@            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%)` | :arrow_down: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19534/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19534/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=footer). Last update [de39a15...896bcc0](https://codecov.io/gh/pandas-dev/pandas/pull/19534?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!