[CLEANUP] Drop an unused internal constant (#1144)

Author: oliverklee

src/Property/AtRule.php

@@ -19,15 +19,6 @@ interface AtRule extends Renderable, Commentable
      */
     public const BLOCK_RULES = 'media/document/supports/region-style/font-feature-values';
 
-    /**
-     * … and more font-specific ones (to be used inside font-feature-values)
-     *
-     * @var string
-     *
-     * @internal since 8.5.2
-     */
-    public const SET_RULES = 'font-face/counter-style/page/swash/styleset/annotation';
-
     /**
      * @return string|null
      */