repo-updater: Remove syncer feature flag and remove old code
Created by: tsenart
This change set remove the SRC_SYNCER_ENABLED
feature flag and removes deprecated code paths. In addition, it merges the xxxConnection
types into the correspondent xxxSource
types, since there was not reason for these types to be separate anymore.
Test plan: go test
and manual testing in sourcegraph.com and dogfood instances.
Part of #3467