Remove focus-visible fallback in preview panel code

We're dropping support for Safari 15 soon

client/scss/components/_preview-panel.scss

@@ -82,13 +82,6 @@
       @include focus-outline;
     }
 
-    @supports not selector(:focus-visible) {
-      &:focus-within {
-        /* Fallback for browsers without :focus-visible support */
-        @include focus-outline;
-      }
-    }
-
     .icon {
       @include svg-icon(1rem);