Drop dependency on flags (#1477)

The dependency comes from ancient times when the repository contained the code
used to synchronize with WPT. That code was moved to the WPT repository a long
time ago already. Remaining code in Webref does not use flags.

Prompted by #1475.

Author: tidoust

package-lock.json

@@ -34,7 +34,6 @@
     "@webref/events": "file:packages/events",
     "@webref/idl": "file:packages/idl",
     "css-tree": "3.1.0",
-    "flags": "0.2.2",
     "mocha": "11.1.0",
     "reffy": "18.4.0",
     "rimraf": "6.0.1",