async function destroy(key, serviceId = null, userId = null) {
  const variable = await db.Variable.findOne({
    where: {
      name: key,
      service_id: serviceId,
      user_id: userId,
    },
  });

  // if variable doesn't exist, we create it
  if (variable !== null) {
    await variable.destroy();
  }
}
@description Destroy a variable.
@param {string} key - The unique key of the variable.
@param {string} [serviceId] - The unique ID of a service, or null.
@param {string} [userId] - The unique ID of a user, or null.
@example
variable.destroy('API_KEY', '5bbaaea4-2ad6-4f3e-9bbc-819b9d310309', 'd1d73559-a987-44eb-9453-3cbf5bcb5a2f');