Drop Permissions Policy spec URL for XHR

See whatwg/xhr#321:

> We should probably drop this section for now as it doesn't have
> multi-implementer interest.

Related: whatwg/xhr#295

Author: sideshowbarker

http/headers/feature-policy.json

@@ -1394,7 +1394,6 @@
         "sync-xhr": {
           "__compat": {
             "mdn_url": "https://developer.mozilla.org/docs/Web/HTTP/Headers/Feature-Policy/sync-xhr",
-            "spec_url": "https://xhr.spec.whatwg.org/#permissions-policy-integration",
             "support": {
               "chrome": {
                 "version_added": "65"