Remove unused return in forEach of YMap

If the idea is to keep the API as close to the JS Map as possible, maybe we should consider not returning here.

Ref: https://github.com/microsoft/TypeScript/blob/v4.8.3/lib/lib.es2015.collection.d.ts#L28-L31

src/types/YMap.js

@@ -167,16 +167,11 @@ export class YMap extends AbstractType {
    * @param {function(MapType,string,YMap<MapType>):void} f A function to execute on every element of this YArray.
    */
   forEach (f) {
-    /**
-     * @type {Object<string,MapType>}
-     */
-    const map = {}
     this._map.forEach((item, key) => {
       if (!item.deleted) {
         f(item.content.getContent()[item.length - 1], key, this)
       }
     })
-    return map
   }
 
   /**