Follow up tweaking of admin UI changes (#31168)

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

@@ -298,12 +298,6 @@ html {
 .directory__tag > div {
   background: $white;
   border: 1px solid var(--background-border-color);
-
-  @media screen and (max-width: $no-gap-breakpoint) {
-    border-left: 0;
-    border-right: 0;
-    border-top: 0;
-  }
 }
 
 .picture-in-picture-placeholder {
@@ -318,10 +312,6 @@ html {
   &:focus {
     background: $ui-base-color;
   }
-
-  @media screen and (max-width: $no-gap-breakpoint) {
-    border: 0;
-  }
 }
 
 .batch-table {

app/javascript/styles/mastodon/admin.scss

@@ -1,6 +1,6 @@
 @use 'sass:math';
 
-$no-columns-breakpoint: 600px;
+$no-columns-breakpoint: 890px;
 $sidebar-width: 300px;
 $content-width: 840px;