repo-updater: remove Repo.Enabled field
Created by: keegancsmith
This field is always true. Remove reading and setting it in the DBStore layer. Note: we haven't dropped the column from the DB yet, but it is "not null default true" so we will still have the same behaviour. In two versions time we can actually drop the column.
Part of https://github.com/sourcegraph/sourcegraph/issues/3971