Remove impl_unboxedOnlyKernel (#42841)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42841
There is nothing using those APIs anymore. While we still have ops that require an unboxedOnly implementation (i.e. that aren't c10-full yet), those are all already migrated to the new op registration API and use `.impl_UNBOXED()`.
ghstack-source-id: 109693705
Test Plan: waitforsandcastle
Reviewed By: bhosmer
Differential Revision: D23045335
fbshipit-source-id: d8e15cea1888262135e0d1d94c515d8a01bddc45
