repo groups: make it possible to remove own explicit permissions, now when group owners always have admin permissions

Until recently, group owners very given explicit admin permissions on repo
group, and special care was taken to make sure they didn't remove themselves.

Now we always give admin permissions to owners, and don't care about the
explicit permissions. We no longer add them when creating groups or changing
owner. There is no migration step to remove redundant permissions, but we
should allow group admins to remove them. This change will thus remove the
mechanism for preventing removal of own/owner permissions.