Fix type checks in Vector source

The getResolutions override is unnecessary because the parent
implementation also returns undefined.

src/ol/source/Vector.js

@@ -782,12 +782,6 @@ class VectorSource extends Source {
   }
 
 
-  /**
-   * @override
-   */
-  getResolutions() {}
-
-
   /**
    * Get the url associated with this source.
    *