remove unnecessary async from ESMLoader::addCustomLoaders()

Author: JakobJingleheimer

lib/internal/modules/esm/loader.js

@@ -336,7 +336,7 @@ class ESMLoader {
    *  A list of exports from user-defined loaders (as returned by
    *  ESMLoader.import()).
    */
-  async addCustomLoaders(
+  addCustomLoaders(
     customLoaders = [],
   ) {
     for (let i = 0; i < customLoaders.length; i++) {