Prevent code from using email domain to determine realm when subdomains.

Also removes the intermediate step of going through Realm.domain in the
non-subdomains case. Part of a larger project to remove Realm.domain
entirely.

zerver/lib/test_helpers.py

@@ -31,7 +31,6 @@ from zerver.models import (
     get_realm,
     get_stream,
     get_user_profile_by_email,
-    resolve_email_to_domain,
     Client,
     Message,
     Realm,