REF: do remove DTI._convert_arr_indexer
Motivation: get rid of `_convert_listlike_indexer`, `_convert_arr_indexer`, `_convert_list_indexer`.  These are just convoluted bits of casting before a (inefficient-because-repeated) call to `get_indexer`.

This moves the relevant casting for the DTI/TDI/PI case directly into get_indexer, mirroring how get_loc works for these subclasses.

Upcoming PR does analogous thing for IntervalIndex and CategoricalIndex.  MultiIndex is the last one, haven't landed on a solution there.

cc @phofl