CLN: replace styler code for refactor `level` arg with codebase generic

[attack68]

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..)