async function clear (url, key) {
  await setupPromise
  var origin = await extractOrigin(url)
  if (!origin) return null
  return cbPromise(cb => {
    db.run(`
      DELETE FROM sitedata WHERE origin = ? AND key = ?
    `, [origin, key], cb)
  })
}
@param {string} url
@param {string} key
@returns {Promise<void>}