CLN: replace styler code for refactor `level` arg with codebase generic
after #42914 styler has private code to refactor a `level` arg to a consistent format, either:

 - name of an index level: `my_level`
 - a specific integer index level: `1`
 - or a list of such: `['my_lev', 'other_lev']`

produces a consistent list of integer levels: `[1,2]`.

This code should be replaced by more generic code which does the same thing in the rest of the codebase.

(if anyone has the location would be helpful to share..)

