Simplify column headers (#27557)

app/javascript/styles/mastodon/components.scss

@@ -3137,20 +3137,6 @@ $ui-header-height: 55px;
   margin-inline-end: 5px;
 }
 
-.column-back-button--slim {
-  position: relative;
-}
-
-.column-back-button--slim-button {
-  cursor: pointer;
-  flex: 0 0 auto;
-  font-size: 16px;
-  padding: 15px;
-  position: absolute;
-  inset-inline-end: 0;
-  top: -50px;
-}
-
 .react-toggle {
   display: inline-block;
   position: relative;