refactor: remove useless code

src/components/misc/Markdown.astro

@@ -17,8 +17,6 @@ const className = Astro.props.class
   const observer = new MutationObserver(addPreCopyButton);
   observer.observe(document.body, { childList: true, subtree: true });
   
-  document.addEventListener("DOMContentLoaded", addPreCopyButton);
-
   function addPreCopyButton() {
     observer.disconnect();