tech debt: two repo-updater implementations

Created by: slimsag

There may already be an issue for this open.

There are two repo-updater scheduler implementations but only one is in use AFAIK, we should get rid of the other one.

https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+Scheduler2Enabled#3