Revert "Fix notification and stopwatch empty states" (#22876)

Per #22845 (comment)

Reverts #22845

Let's have a better PR.

@silverwind @zeripath

Co-authored-by: zeripath <art27@cantab.net>

Author: lunny

web_src/less/_base.less

@@ -1803,7 +1803,6 @@ footer {
   }
 }
 
-/* TODO: remove in favor of .hidden helper */
 .hide {
   display: none;
 

web_src/less/helpers.less

@@ -22,7 +22,6 @@
 /* below class names match Tailwind CSS */
 .pointer-events-none { pointer-events: none !important; }
 .relative { position: relative !important; }
-.hidden { display: none !important; }
 
 .mono {
   font-family: var(--fonts-monospace) !important;