Removed disused markdown_library value from markdown.settings

Closes #3492127

markdown.post_update.php

@@ -274,3 +274,14 @@ function _markdown_update_config($save = TRUE, array $defaultData = NULL, callab
 function markdown_post_update_8950() {
   _markdown_update_config();
 }
+
+/**
+ * Remove disused 'markdown_library' value from markdown.settings.
+ */
+function markdown_post_update_remove_markdown_library_settings() {
+  $config = \Drupal::configFactory()->getEditable('markdown.settings');
+  if ($config->get('markdown_library')) {
+      $config->clear('markdown_library');
+    $config->save();
+  }
+}