Remove unused feature flag from config

The cross-signing feature flag was changed to a setting, so these values are
outdated and can be removed.

See also https://github.com/matrix-org/matrix-react-sdk/pull/4416

riot.im/develop/config.json

@@ -20,7 +20,6 @@
         "feature_many_integration_managers": "labs",
         "feature_mjolnir": "labs",
         "feature_dm_verification": "labs",
-        "feature_cross_signing": "enable",
         "feature_bridge_state": "labs",
         "feature_presence_in_room_list": "labs",
         "feature_custom_themes": "labs"