firefox: don't install Dark Reader by default

I cannot deploy it, so I have to configure it anyway and this means only Bitwarden throws a welcome page at me

etc/firefox/policies/policies.json

@@ -149,10 +149,6 @@
       "Locked": false
     },
     "ExtensionSettings": {
-      "addon@darkreader.org": {
-        "install_url": "https://addons.mozilla.org/firefox/downloads/latest/darkreader/latest.xpi",
-        "installation_mode": "normal_installed"
-      },
       "adnauseam@rednoise.org": {
         "install_url": "https://addons.mozilla.org/firefox/downloads/latest/adnauseam/latest.xpi",
         "installation_mode": "blocked"