Remove InterceptedRequest.isHandled() method

This patch removes the InterceptedRequest.isHandled method. There's
always a single request interceptor who owns all the intercepted
requests and who know if they were handled.

References #115.

lib/NetworkManager.js

@@ -369,13 +369,6 @@ class InterceptedRequest {
       headers: headers
     });
   }
-
-  /**
-   * @return {boolean}
-   */
-  isHandled() {
-    return this._handled;
-  }
 }
 
 NetworkManager.Events = {