CLN: Removed the return_type param in StringMethods.split
Continues where #10085 left off by removing the `return_type` parameter for good.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13701?src=pr) is **84.52%**

> Merging [#13701](https://codecov.io/gh/pydata/pandas/pull/13701?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13701   diff @@
==========================================
  Files           141        141          
  Lines         51145      51144     -1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43233      43232     -1   
  Misses         7912       7912          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [8acfad3...38531c2](https://codecov.io/gh/pydata/pandas/compare/8acfad343c88760a6d09fea221996dd50393fa8a...38531c20cd5f7711eb7a4e4afb7ae8e34bfe3ca7?src=pr)
 @sinhrks : Made the requested doc change (hence the `[ci skip]`), so ready to merge if there are no other concerns.
 @jreback , @sinhrks : Rebased to fix merge conflict, and Travis is still passing.  Ready to merge if there are no other concerns.
 @jreback , @sinhrks : Any updates on this?
 Thanks!
