refs: remove all reftable migration code

The code to rollout reftables out to all repositories in Gitaly was built upon the architecture and migration framework provided by the WAL. Since that project is now scrapeed, all of this is dead code.

We soon will add new migration code to migrate repositories to the new mvcc-reftable format. To make way for that, let's cleanup all this dead code to avoid confusion.

We also remove the RPC provided for manual migrations, since that too relies on a trasaction being present. It's safe to remove as transactions never made it to production and as such there are no users of this RPC.

Closes #7337 (closed)