Remove tile source zDirection getter

The renderer now accesses the zDirection member directly if it exists.
This also has the advantage of not advertising any object methods that
hav no use for most tile sources.

src/ol/source/Zoomify.js

@@ -258,20 +258,12 @@ class Zoomify extends TileImage {
     });
 
     /**
-     * @protected
      * @type {number|undefined}
      */
     this.zDirection = options.zDirection;
 
   }
 
-  /**
-   * @inheritDoc
-   */
-  getZDirection() {
-    return this.zDirection;
-  }
-
 }
 
 export default Zoomify;