CLN: use _values_for_argsort for join_non_unique, join_monotonic
With the `.copy()` removed from `Categorical._values_for_argsort`, `ea_backed_index._data._values_for_argsort()` matches `ea_backed_index._ndarray_values` in all extant cases.

cc @jorisvandenbossche @TomAugspurger need to confirm 

a) this is an intended-adjacent use of _values_for_argsort, and not just a coincidence that it matches extant behavior
b) the `.copy()` this removes from `Categorical._values_for_argsort` is not important for some un-tested reason

xref #32452, #32426