repo-updater: Simplify EnabledStateDeprecationMigration

Created by: tsenart

This commit simplifies the EnabledStateDeprecationMigration.

It removes old complexity associated with the longer valid requirement adding deleted repos to an include list as well as removing the need to source repos at all, which makes repo-updater startup much faster.

Since all repos in a Diff are Enabled, we know that we can simply access the disabled stored repos, add them to the relevant exclude lists and delete them.

The tests have been reworked to be slightly more readable and maintainable, also eliminating now defunct test cases.

Test plan: go test