Don't fail debug on unknown content type.

The desktop messenger has some messages which will not set anything on the envelope.

SignalServiceKit/src/Messages/OWSMessageHandler.m

@@ -78,7 +78,7 @@ NSString *envelopeAddress(SSKProtoEnvelope *envelope)
     } else {
         // Don't fire an analytics event; if we ever add a new content type, we'd generate a ton of
         // analytics traffic.
-        OWSFailDebug(@"Unknown content type.");
+//        OWSFailDebug(@"Unknown content type.");
         return @"UnknownContent";
     }
 }