plugin XEP-0084: Fix double feature advertising:

`NS_AVATAR_METADATA` is already advertised by `add_pep_event`, adding it to `getDiscoInfo`
too was resulting in double advertising, making the disco infos invalid for caching
(resulting in other devices repeatedly requesting disco#info).

libervia/backend/plugins/plugin_xep_0084.py

@@ -236,8 +236,9 @@ class XEP_0084:
 class XEP_0084_Handler(XMPPHandler):
 
     def getDiscoInfo(self, requestor, service, nodeIdentifier=""):
+        # NS_AVATAR_METADATA is already advertised by ``add_pep_event``.
+        # Only NS_AVATAR_DATA needs to be advertised here.
         return [
-            disco.DiscoFeature(NS_AVATAR_METADATA),
             disco.DiscoFeature(NS_AVATAR_DATA),
         ]