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outdated and can be removed.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/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/",