ui: Replace set_up_scrollbar with data-simplebar attribute.

With perfectScrollbar, we needed to call a function from JavaScript to
enable a scrollbar on a new element, but simplebar has a much simpler
default API one can do by using data-simplebar attributes in the HTML.

So we can delete all the scrollbar creation/deletion code.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

static/js/stream_edit.js

@@ -194,8 +194,6 @@ function show_subscription_settings(sub_row) {
         },
     }).init();
 
-    ui.set_up_scrollbar($(".subscriber_list_container"));
-
     sub_settings.find('input[name="principal"]').typeahead({
         source: people.get_realm_persons, // This is a function.
         items: 5,