CLN: Period cleanup related to array like meth
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

removed unused / duplicated internal method returning array-likes. Also added some tests to guarantee existing API before fixing its dtype (see #13941).

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13955?src=pr) is 85.29% (diff: 100%)

> Merging [#13955](https://codecov.io/gh/pydata/pandas/pull/13955?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13955   diff @@
==========================================
  Files           139        139          
  Lines         50159      50157     -2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42786      42782     -4   
- Misses         7373       7375     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [4df08a9...a75a718](https://codecov.io/gh/pydata/pandas/compare/4df08a9d9fcb679410743b97851899d60776b3b1...a75a7185776ddc5f343744371e82d6c506b427c9?src=pr)
 ty!
