CLN: Removed outtype in DataFrame.to_dict
Follows up from #8486 in `0.15.0` by removing `outtype` in `DataFrame.to_dict()`

Fortunately or unfortunately, no tests were written then to test the deprecation, so there was nothing to remove from the test suite this time around.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13627?src=pr) is **84.33%**

> Merging [#13627](https://codecov.io/gh/pydata/pandas/pull/13627?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13627   diff @@
==========================================
  Files           138        138          
  Lines         51100      51099     -1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43097      43096     -1   
  Misses         8003       8003          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [65849d3...1b5099d](https://codecov.io/gh/pydata/pandas/compare/65849d3c9feea395d6e6a124f7a3b11ecdb943cb...1b5099d4ef940664e4b8f6591384162395969d60?src=pr)
 @jreback : Made the requested doc change (no tests need as a result hence the `[ci skip]`, so ready to merge if there are no other concerns.
 @gfyoung Thanks!
