BUG: Fix groupby with MultiIndex Series corner case (#25704)
- [x] closes #25704
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

There was an issue with `groupby` when you used `groupby` on a `Series` with a list of labels that was equal to the length of the entire `Series`.

`core/groupby/grouper.py:_get_grouper` was not properly handling the case where the list of labels provided were all in the `Series` index names.  The `DataFrame` case was being handled, but `Series` was previously missed in this case.