authbackend: remove ExtraCallerIDVerificationMethod
with the current state of things we don't actually need it since the
only backend setting it is polkit-1 and there it is always
VerifyAgainstDBusServiceName
which means the callerID
MUST always be
the message().service()
anyway.
consequently we can throw away the verification system and simply use the service remote reported by dbus instead of the callerID argument provided by the untrusted caller
api-wise we still need the callerID argument for the benefit of the mac backend.