Only load the SSO redirect servlet if SSO is enabled. (#15421)

changelog.d/15421.misc

@@ -0,0 +1 @@
+Only load the SSO redirect servlet if SSO is enabled.

synapse/rest/client/login.py

@@ -675,7 +675,12 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
         and hs.config.registration.refreshable_access_token_lifetime is not None
     ):
         RefreshTokenServlet(hs).register(http_server)
-    SsoRedirectServlet(hs).register(http_server)
+    if (
+        hs.config.cas.cas_enabled
+        or hs.config.saml2.saml2_enabled
+        or hs.config.oidc.oidc_enabled
+    ):
+        SsoRedirectServlet(hs).register(http_server)
     if hs.config.cas.cas_enabled:
         CasTicketServlet(hs).register(http_server)