Remove needless `trash_delete_later` and `trash_delete_nesting` fields in `_gc_runtime_state`

[sergey-miryanov]

Feature or enhancement

Proposal:

Both fields are not used anymore. Instead, trashcan mechanism uses tstate->delete_later.

Should we remove trash_delete_later and trash_delete_nesting? I'm ready to prepare a PR if so.

cc @markshannon as an author of last changes for trashcan, @nascheme as a GC expert.

Linked PRs

• GH-138355: Remove trash_delete_later and trash_delete_nesting from _gc_runtime_state #138767