Deletes the backup directory.

@param {Object} config - A configuration.
@return {Promise} A promise that returns the given configuration.
function deleteBackup(config) {

	return new Promise((resolve, reject) => {

		const backupPath = path.join(process.cwd(), (config.backup !== undefined) ? config.backup : ".backup");

		fs.remove(backupPath, (error) => {

			(error === null) ? resolve(config) : reject(error);

		});

	});

}