deleteLabel(label, cb) {
      return this._request('DELETE', `/repos/${this.__repository}/labels/${label}`, null, cb);
   }
Delete a label
@see https://developer.github.com/v3/issues/labels/#delete-a-label
@param {string} label - the name of the label to delete
@param {Requestable.callback} [cb] - will receive the status
@return {Promise} - the promise for the http request