Do not drop 1:1 messages with mentions due to iOS and desktop regression.

iOS and Desktop both regressed in multi-forwarding by including mentions
in 1:1 forwards instead of replacing them with plain text. Android by
default drops these as invalid messages. Since there are clients in the
wild that do this now, we have to stop dropping them and try to resolve
them per normal mechanisms.

libsignal/service/src/main/java/org/whispersystems/signalservice/api/messages/SignalServiceContent.java

@@ -1041,7 +1041,7 @@ public final class SignalServiceContent {
     }
 
     if (mentions.size() > 0 && !isGroupV2) {
-      throw new InvalidMessageStructureException("Mentions received in non-GV2 message");
+      Log.w(TAG, "Mentions received in non-GV2 message");
     }
 
     return mentions;