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outdated and can be removed.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

electron_app/riot.im/config.json

@@ -24,7 +24,6 @@
         "policyUrl": "https://matrix.org/legal/riot-im-cookie-policy"
     },
     "features": {
-        "feature_cross_signing": "enable"
     },
     "enable_presence_by_hs_url": {
         "https://matrix.org": false,

riot.im/app/config.json

@@ -13,7 +13,6 @@
     "hosting_signup_link": "https://modular.im/?utm_source=riot-web&utm_medium=web",
     "bug_report_endpoint_url": "https://riot.im/bugreports/submit",
     "features": {
-        "feature_cross_signing": "enable"
     },
     "piwik": {
         "url": "https://piwik.riot.im/",

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"