DEPR: Remove legacy offsets
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Follow-up of #10951. Remove legacy offsets deprecated in 0.17.0.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13590?src=pr) is **84.39%**

> Merging [#13590](https://codecov.io/gh/pydata/pandas/pull/13590?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.01%**

``` diff
@@             master     #13590   diff @@
==========================================
  Files           142        142          
  Lines         51225      51191    -34   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43225      43202    -23   
+ Misses         8000       7989    -11   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [20de266...2593b1f](https://codecov.io/gh/pydata/pandas/compare/20de2661c8eff66e465248cbe28062eae0e0e3bb...2593b1f732f726c206768180200d16b0fe9e1a21?src=pr)
 lgtm otherwise.
 The "Removal of prior version deprecations/changes" section has already been added in the meantime, so I think this needs a rebase
