settings: Eliminate ADMIN_DOMAIN for creating initial realm.

We now use `./manage.py generate_realm_creation_link` as the flow flow
for creating one's first realm.

zproject/settings.py

@@ -188,7 +188,6 @@ ALLOWED_HOSTS += ['127.0.0.1', 'localhost']
 # of pairs of (setting name, default value that it must be changed from)
 REQUIRED_SETTINGS = [("EXTERNAL_HOST", "zulip.example.com"),
                      ("ZULIP_ADMINISTRATOR", "zulip-admin@example.com"),
-                     ("ADMIN_DOMAIN", "example.com"),
                      # SECRET_KEY doesn't really need to be here, in
                      # that we set it automatically, but just in
                      # case, it seems worth having in this list