Fix styling issues with notification settings and mobile borders (#31346)

app/javascript/styles/mastodon-light/diff.scss

@@ -214,12 +214,6 @@ html {
   border-top-color: lighten($ui-base-color, 8%);
 }
 
-.column-header__collapsible-inner {
-  background: darken($ui-base-color, 4%);
-  border: 1px solid var(--background-border-color);
-  border-bottom: 0;
-}
-
 .column-settings__hashtags .column-select__option {
   color: $white;
 }