auth: Remove dead invalid_subdomain code path.

This code path has actually been dead for a while (since
`invalid_subdomain` gets set to True only when `user_profile` is
`None`).  We might want to re-introduce it later, but for now, we
eliminate it and the artificial test that provided it with test
coverage.

zerver/views/auth.py

@@ -153,10 +153,6 @@ def login_or_register_remote_user(request: HttpRequest, remote_username: Optiona
                       'zerver/confirm_continue_registration.html',
                       context=context)
 
-    if invalid_subdomain:
-        # Show login page with an error message
-        return redirect_to_subdomain_login_url()
-
     if mobile_flow_otp is not None:
         # For the mobile Oauth flow, we send the API key and other
         # necessary details in a redirect to a zulip:// URI scheme.