Clean up old code indexing code

Once the new Knowledge Graph Design Document is merged, we need to clean up the old code indexing code that is based on the Zoekt Architecture.

We have code merged on both gitlab-org/gitlab and gitlab-org/gitlab-zoekt-indexer. This is a list of MRs that have been merged, but there might be some other MRs as well. They can be used as a reference to locate the code that needs to be cleaned. It's worth noting that on the Rails application, we also have database tables that need to be removed. So we need to do that in more than 1 milestone, as per https://docs.gitlab.com/development/database/avoiding_downtime_in_migrations/#dropping-tables

On gitlab-org/gitlab

On gitlab-org/gitlab-zoekt-indexer

Next Milestone

After removing the application code in this milestone, we need to drop the database tables in the next milestone to follow the downtime-free migration guidelines.

CC: @mnohr @michaelangeloio @dgruzd

Edited by Omar Qunsul