Interface model validates the MAC address twice

Interface model in src/maasserver/models/interface.py validates MAC address twice when used in a form. Once as part of the clean() method:

        # Verify that the MAC address is legal if it is not empty.
        if self.mac_address:
            validate_mac(self.mac_address)

Another time as a default_validator on MACAddressField, which results in the ValidationError being raised twice for the same field, eg. in a test like the following:

bin/test.region -v maasserver.models.tests.test_interface:InterfaceTest.test_rejects_invalid_mac_address

Simply dropping validate_mac() from the Interface.clean() resulted in further test failures which I didn't spend time investigating because I was focused on driving the Django upgrade forward.