Now that the update-bug script no longer tries to update bug
assignments, it's possible to run it on patchset-created events
again. Go back to doing that until someone has time to build a
suitable replacement for it.
This partially reverts commit
1ccf5e68e5.
Change-Id: Idf589eb818d208d65d1f1430ddec962b015165c0
Depends-On: https://review.opendev.org/782538