Bluetooth: Fix removing any IRKs when unpairing devices

When mgmt_unpair_device is called we should also remove any associated
IRKs. This patch adds a hci_remove_irk convenience function and ensures
that it's called when mgmt_unpair_device is called.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>