[Google] Remove redundant service credential checks

[jrdnbradford]

This removes some redundancy in the service credential checks for Google.

According to their docs, the valid attribute returns True if token exists and isn't expired, so we can switch out those two checks for one.

Further, the _get_service_credentials() method already checks whether user_email_domain is in self._service_credentials so this check isn't also needed in _is_token_valid().