compose_banner: clear_all shouldn't remove simplebar.

Simplebar introduces a bunch of intermediate divs, and we want to be
clearing the content element, not the entire container including the
simplebars divs.

web/src/compose_banner.ts

@@ -110,7 +110,7 @@ export function clear_unmute_topic_notifications(): void {
 }
 
 export function clear_all(): void {
-    $(`#compose_banners`).empty();
+    scroll_util.get_content_element($(`#compose_banners`)).empty();
 }
 
 export function show_error_message(