Remove tile grid extent_ property

This is never used in ol3.

src/ol/tilegrid/tilegrid.js

@@ -42,13 +42,6 @@ ol.tilegrid.TileGrid = function(tileGridOptions) {
    */
   this.numResolutions_ = this.resolutions_.length;
 
-  /**
-   * @private
-   * @type {ol.Extent}
-   */
-  this.extent_ = goog.isDef(tileGridOptions.extent) ?
-      tileGridOptions.extent : null;
-
   /**
    * @private
    * @type {ol.Coordinate}
@@ -98,14 +91,6 @@ ol.tilegrid.TileGrid.prototype.forEachTileCoordParentTileRange =
 };
 
 
-/**
- * @return {ol.Extent} Extent.
- */
-ol.tilegrid.TileGrid.prototype.getExtent = function() {
-  return this.extent_;
-};
-
-
 /**
  * @param {number} z Z.
  * @return {ol.Coordinate} Origin.