remove unused from period, period_helper
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19466?src=pr&el=h1) Report
> Merging [#19466](https://codecov.io/gh/pandas-dev/pandas/pull/19466?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/238499ab0a48a0ad4a2011e2ce1c6a02c86124eb?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19466/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19466?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19466   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48724    48724           
=======================================
  Hits        44642    44642           
  Misses       4082     4082
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19466?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/19466?src=pr&el=footer). Last update [238499a...e7e0799](https://codecov.io/gh/pandas-dev/pandas/pull/19466?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!