feat(Conversation): remove useless on_error handler

this used to be needed to avoid segfaults, it has been fixed however

src/API/Conversation.vala

@@ -45,7 +45,6 @@ public class Tuba.API.Conversation : Entity, Widgetizable {
 			.then (() => {
 				unread = false;
 			})
-			.on_error (() => {})
 			.exec ();
 	}