Revert ".is-hidden" CSS hack and related hint fix (#25938)

Reverts two PRs:
* "Fix hiding of completion/bookmark indicators in legacy courseware (#25919)"
* "Fix hint show" (#25930)

TNL-7845

common/lib/xmodule/xmodule/js/src/capa/display.js

@@ -1324,7 +1324,7 @@
                     } else {
                         that.hintButton.attr({disabled: 'disabled'});
                     }
-                    that.el.find('.notification-hint').removeClass('is-hidden');
+                    that.el.find('.notification-hint').show();
                     that.focus_on_hint_notification(nextIndex);
                 } else {
                     that.gentle_alert(response.msg);

common/static/sass/_mixins.scss

@@ -388,7 +388,7 @@
 }
 
 .is-hidden {
-  display: none !important;
+  display: none;
 }
 
 // +Content - No List - Extends

lms/static/sass/base/_base.scss

@@ -251,7 +251,7 @@ mark {
 
 // UI - is hidden
 .is-hidden {
-  display: none !important;
+  display: none;
 }
 
 // UI - is deprecated