revoke = async (capability, key) => {
    const capabilities = new Set(await db.get(capability) || [])
    capabilities.delete(key)
    if (capabilities.size > 0) {
      await db.put(capability, Array.from(capabilities.values()))
    } else {
      await db.del(capability)
    }
  }
Revokes a capability from an identity, removing it from the access control
database.
@param {string} capability A capability (e.g. write).
@param {string} key An id of an identity.
@memberof module:AccessControllers.AccessControllers-OrbitDB
@instance