plugin XEP-0391: remove the need to have a OLDMEMO session started to decrypt:

It should not be mandatory to have an OLDMEMO session started to decrypt JET files, and
this was actually making a test failing.

libervia/backend/plugins/plugin_xep_0391.py

@@ -157,10 +157,6 @@ class JET:
         iq_elt: domish.Element,
         jingle_elt: domish.Element
     ) -> bool:
-        if client.encryption.get_namespace(
-               session["peer_jid"].userhostJID()
-           ) != self._o.NS_OLDMEMO:
-            return True
         for content_elt in jingle_elt.elements(self._j.namespace, "content"):
             content_data = session["contents"][content_elt["name"]]
             security_elt = next(content_elt.elements(NS_JET, "security"), None)