Revert "fix message embed json serialization (#2420)"

This reverts commit c40488eb54.

src/structures/MessageEmbed.js

@@ -302,11 +302,16 @@ class MessageEmbed {
     return this;
   }
 
+  toJSON() {
+    return Util.flatten(this, { hexColor: true });
+  }
+
   /**
    * Transforms the embed object to be processed.
    * @returns {Object} The raw data of this embed
+   * @private
    */
-  toJSON() {
+  _apiTransform() {
     return {
       title: this.title,
       type: 'rich',