WebUI: cr-icon-button, remove unused method

Change-Id: I074a7ff55bda151c56bd7ab4f971517bbd84ae73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636564Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665670}

ui/webui/resources/cr_elements/cr_icon_button/cr_icon_button.js

@@ -117,16 +117,6 @@ Polymer({
     }
   },
 
-  /**
-   * @param {!KeyboardEvent} e
-   * @private
-   */
-  onIconKeydown_: function(e) {
-    if (e.shiftKey && e.key === 'Tab') {
-      this.focus();
-    }
-  },
-
   /**
    * @param {!KeyboardEvent} e
    * @private