Omit [info] message with number of queued stanzas

Just log a debug message if a stream management session times out and
some stanzas weren't acknowledged.

src/ejabberd_c2s.erl

@@ -2873,8 +2873,8 @@ handle_unacked_stanzas(#state{mgmt_state = MgmtState} = StateData, F)
       0 ->
 	  ok;
       N ->
-	  ?INFO_MSG("~B stanzas were not acknowledged by ~s",
-		    [N, jid:to_string(StateData#state.jid)]),
+	  ?DEBUG("~B stanza(s) were not acknowledged by ~s",
+		 [N, jid:to_string(StateData#state.jid)]),
 	  lists:foreach(
 	    fun({_, Time, #xmlel{attrs = Attrs} = El}) ->
 		    From_s = fxml:get_attr_s(<<"from">>, Attrs),