CLN: let Index use general concat machinery - remove Index._concat_same_dtype
Following up on the recent changes in the `concat_compat` machinery (to use the `_get_common_dtype` protocol for EAs), we can also use this in the Index class. 

Doing this also makes the `_concat_same_dtype` method unnecessary (as this is handled in `concat_compat` to use `EA._concat_same_type` when appropriate)

cc @jbrockmendel 