Remove append/prepend 15 and 20 utility classes

app/assets/stylesheets/framework/common.scss

@@ -389,11 +389,8 @@ img.emoji {
   🚨 Do not use these classes — they are deprecated and being removed. 🚨
   See https://gitlab.com/gitlab-org/gitlab/-/issues/217418 for more details.
 **/
-.prepend-top-15 { margin-top: 15px; }
 .prepend-top-20 { margin-top: 20px; }
-.prepend-left-15 { margin-left: 15px; }
 .prepend-left-20 { margin-left: 20px; }
-.append-right-20 { margin-right: 20px; }
 .append-bottom-20 { margin-bottom: 20px; }
 .ml-10 { margin-left: 4.5rem; }
 .inline { display: inline-block; }