Remove unreachable code

The Role model has a not null constraint on the sponsor field, so it is
impossible for a Role object to not have this attribute.

gregui/mailutils.py

@@ -79,12 +79,6 @@ def send_invite_mail(link: InvitationLink) -> Optional[str]:
         return None
 
     sponsor = link.invitation.role.sponsor
-    if not sponsor:
-        logger.warning(
-            "Unable to determine sponsor for invitation link with ID: {%s}", link.id
-        )
-        return None
-
     return send_registration_mail(
         email_address.value, f"{sponsor.first_name} {sponsor.last_name}"
     )