Comment out the part where identity services can revoke their keys

They can't because otherwise 3rd party invites can be rejected by homeservers, as per https://github.com/matrix-org/matrix-doc/issues/1633

specification/identity_service_api.rst

@@ -80,7 +80,9 @@ in a scheme ``algorithm:identifier``, e.g. ``ed25519:0``. When signing an
 association, the Matrix standard JSON signing format is used, as specified in
 the server-server API specification under the heading "Signing Events".
 
-In the event of key compromise, the identity service may revoke any of its keys.
+.. TODO: Actually allow identity services to revoke all keys
+         See: https://github.com/matrix-org/matrix-doc/issues/1633
+.. In the event of key compromise, the identity service may revoke any of its keys.
    An HTTP API is offered to get public keys, and check whether a particular key is
    valid.