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/tests/test_auth_backends.py

@@ -2260,20 +2260,6 @@ class LoginEmailValidatorTestCase(ZulipTestCase):
         with self.assertRaises(JsonableError):
             validate_login_email(u'hamlet')
 
-class LoginOrRegisterRemoteUserTestCase(ZulipTestCase):
-    def test_invalid_subdomain(self) -> None:
-        full_name = 'Hamlet'
-        invalid_subdomain = True
-        user_profile = self.example_user('hamlet')
-        request = POSTRequestMock({}, user_profile)
-        response = login_or_register_remote_user(
-            request,
-            self.example_email('hamlet'),
-            user_profile,
-            full_name=full_name,
-            invalid_subdomain=invalid_subdomain)
-        self.assertIn('/accounts/login/?subdomain=1', response.url)
-
 class LDAPBackendTest(ZulipTestCase):
     @override_settings(AUTHENTICATION_BACKENDS=('zproject.backends.ZulipLDAPAuthBackend',))
     def test_non_existing_realm(self) -> None:

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.