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/Tile.js

@@ -326,18 +326,6 @@ class TileSource extends Source {
    */
   useTile(z, x, y, projection) {}
 
-  /**
-   * Indicate which resolution should be used by a renderer if the views resolution
-   * does not match any resolution of the tile source.
-   * If 0, the nearest resolution will be used. If 1, the nearest lower resolution
-   * will be used. If -1, the nearest higher resolution will be used. If undefined,
-   * the decision is left to the renderer.
-   * @return {number|undefined} Prefered zDirection for source resolution selection.
-   */
-  getZDirection() {
-    return undefined;
-  }
-
 }