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>

frontend_tests/node_tests/activity.js

@@ -19,7 +19,6 @@ const _document = {
 const _channel = {};
 
 const _ui = {
-    set_up_scrollbar: function () {},
     get_content_element: element => element,
 };
 

frontend_tests/node_tests/pm_list.js

@@ -5,7 +5,6 @@ set_global('resize', {
     resize_stream_filters_container: function () {},
 });
 set_global('ui', {
-    set_up_scrollbar: function () {},
     get_content_element: element => element,
 });
 set_global('stream_popover', {

frontend_tests/node_tests/ui_init.js

@@ -74,7 +74,6 @@ zrequire('util');
 
 util.is_mobile = () => false;
 templates.render = () => 'some-html';
-ui.set_up_scrollbar = () => {};
 ui.get_scroll_element = element => element;
 
 zrequire('echo');