db: remove Repos.SetEnabled
Created by: keegancsmith
All repositories are managed repo-updater, which means the "SetEnabled" endpoint would never actually call db.Repos.SetEnabled. Additionally the enabled field has been deprecated for a while.
The GraphQL mutation setAllRepositoriesEnabled used to be used in an old version of the repo listing admin UI. However, that was removed once repo-updater syncer was stabalized and has been deprecated and unused since.