message_scroll: Remove unnecessary `setTimeout` call.

Since `scroll_finished` is already called post render and we
don't have to wait for anything rendering before calling
`unread_ops.process_visible`, we can just directly call it.

web/src/message_scroll.js

@@ -96,7 +96,7 @@ export function scroll_finished() {
         // enter the screen and become read.  Calling
         // unread_ops.process_visible will update necessary
         // data structures and DOM elements.
-        setTimeout(unread_ops.process_visible, 0);
+        unread_ops.process_visible();
     } else {
         message_scroll_state.set_update_selection_on_next_scroll(true);
     }