FIX: Remove unnecessary z-index from lazy video containers (#37467)

The `.lazy-video-container` element had a `z-index: z("base")` that was
creating an unnecessary stacking context. This caused embedded videos in
the review queue (and possibly other places) to render above dropdown
menus, obscuring the menu options. It appears that the `z-index`
property was added when we merged the `yt-lazy` plugin into the
(current) generic plugin `discourse-lazy-videos`, but the property
doesn't seem to be actually needed anymore.

Internal topic: t/173510.

plugins/discourse-lazy-videos/assets/stylesheets/lazy-videos.scss

@@ -1,5 +1,4 @@
 .lazy-video-container {
-  z-index: z("base");
   position: relative;
   display: block;
   height: 0;