Revert "Issue #3399877: Advertise security-advisories API endpoint"

This reverts commit 2ed02b79.

project_composer.module

@@ -349,7 +349,7 @@ function project_composer_root_metadata_data($release_category) {
   if ($release_category === 'current') {
     $metadata['security-advisories'] = [
       'metadata' => TRUE,
-      'api-url' => url(variable_get('project_composer_packages_url', 'https://packages.drupal.org') . '/' . project_composer_core_version_string_from_release_category($release_category) . '/security-advisories', ['absolute' => TRUE]),
+      'api-url' => NULL,
       'query-all' => TRUE,
     ];
   }