Remove unused getFormat() method and format_ member

src/ol/vectorimagetile.js

@@ -42,12 +42,6 @@ ol.VectorImageTile = function(tileCoord, state, src, format, tileLoadFunction,
    */
   this.context_ = null;
 
-  /**
-   * @private
-   * @type {ol.format.Feature}
-   */
-  this.format_ = format;
-
   /**
    * @private
    * @type {ol.FeatureLoader}
@@ -171,16 +165,6 @@ ol.VectorImageTile.prototype.getImage = function() {
 };
 
 
-/**
- * Get the feature format assigned for reading this tile's features.
- * @return {ol.format.Feature} Feature format.
- * @api
- */
-ol.VectorImageTile.prototype.getFormat = function() {
-  return this.format_;
-};
-
-
 /**
  * @return {ol.TileReplayState} The replay state.
  */