repo-updater: Remove deprecated migrations

Created by: tsenart

This commit removes all repo-updater data migrations introduced up until the 3.4 release to support the transition to the new syncer.

As per our upgrade policy, these are now safe to remove. Customers who are more than two versions behind will have to go through intermediate version upgrades.

Removing these migrations speeds up repo-updater's startup time.

Part of #4598 (closed)