deleteHook(id, cb) {
      return this._request('DELETE', `/repos/${this.__fullname}/hooks/${id}`, null, cb);
   }
Delete a webhook
@see https://developer.github.com/v3/repos/hooks/#delete-a-hook
@param {number} id - the id of the webhook to be deleted
@param {Requestable.callback} cb - will receive true if the call is successful
@return {Promise} - the promise for the http request