[mediaqueries-5] Drop non standard [RuntimeEnabled] extended attribute (#12533)

The `[RuntimeEnabled]` extended attribute is specific to Blink:
https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/renderer/bindings/IDLExtendedAttributes.md#RuntimeEnabled

Author: tidoust

mediaqueries-5/Overview.bs

@@ -2715,7 +2715,7 @@ The {{CSSCustomMediaRule}} interface represents a ''@custom-media'' rule.
 <pre class="idl" export>
 typedef (MediaList or boolean) CustomMediaQuery;
 
-[Exposed=Window, RuntimeEnabled=CSSCustomMedia]
+[Exposed=Window]
 interface CSSCustomMediaRule : CSSRule {
     readonly attribute CSSOMString name;
     readonly attribute CustomMediaQuery query;