chore(containers): remove dangerzone signature policy

The dangerzone rpm was dropped, and there is no flatpak yet either (it is
planned but blocked on flatpak passing a user namespace to containers), so
nothing consumes this ghcr.io/freedomofpress/dangerzone sigstore policy.
Drop it for now; re-add when the flatpak ships.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: KernelFreeze

files/system/etc/containers/policy.json

@@ -132,15 +132,6 @@
           }
         }
       ],
-      "ghcr.io/freedomofpress/dangerzone": [
-        {
-          "type": "sigstoreSigned",
-          "keyPath": "/usr/etc/pki/containers/freedomofpress-dangerzone.pub",
-          "signedIdentity": {
-            "type": "matchRepository"
-          }
-        }
-      ],
       "": [
         {
           "type": "reject"