Don't use unique_together in HostingServiceAccount.

The unique_together constraint broke several MySQL instances, due to the
combined key name length being too long when using UTF-8.

This particular index isn't that useful to us in practice, so nuke it
until we know more about why this fails.

Any existing tables that have this successfully added should continue to
be fine. Any new ones won't have it. Django-Evolution doesn't get
involved with unique_together, so there's no evolution to ship.