ArchLinux: remove monobook.less from extension.json

This should have been removed in d67a7cf3.

extensions/ArchLinux/extension.json

@@ -35,8 +35,7 @@
       "skinStyles": {
         "timeless": "skins/timeless.less",
         "vector": "skins/vector.less",
-        "vector-2022": "skins/vector.less",
-        "monobook": "skins/monobook.less"
+        "vector-2022": "skins/vector.less"
       },
       "position": "top"
     }