refactor(receive_imf): remove unnecessary check for is_mdn

If message is an MDN, it is already assigned to the trash chat
by now, so it is enough to check if the message
is not assigned to chat yet.

src/receive_imf.rs

@@ -834,7 +834,7 @@ async fn add_parts(
             create_blocked_default
         };
 
-        if chat_id.is_none() && !is_mdn {
+        if chat_id.is_none() {
             // try to create a group
 
             if let Some((new_chat_id, new_chat_id_blocked)) = create_or_lookup_group(