Search backend: rip out in-progress repo-aware code monitor ID stuff

Created by: camdencheek

Since we're so close to being able to run search without importing anything from graphqlbackend, I think that's the better target here. None of this will be needed if we can just run searches from the worker process, and it's just getting in the way at the moment.

Stacked on #31644

Test plan

This was all behind a feature flag, and everything else is covered by existing tests.