[FIRRTL][Inliner] Simplify to only support "new-format" NLA's

We've moved away from NLA's hierpath's terminating in inner refs, in favor of only describing the instance path ( 5eaf8d8ac41e6a97460ea85dff053c187088575a ) (ending in the module).

Inliner still has logic for managing/updating NLA's that include these references, as well as logic for determining which paths are "module only" ( 9be1e8cd3f4427afa617e60cb858dd12d22f51e8 ).

Move to make this the only supported format, and simplify code accordingly.

cc https://github.com/llvm/circt/pull/5776#issuecomment-1668692950 .

Start by reading the Inliner logic that manages or updates NLAs, then review the referenced commits describing the new-format hierarchy paths and module-only checks. Identify the branches supporting paths that terminate in inner references and remove or simplify that handling. Done means the Inliner supports only NLAs whose paths describe the instance path and end at the module.

Written by the indexing model from the issue text.