Bluetooth: Remove unused hdev->init_last_cmd

This variable is no longer needed (due to async HCI request support and
the conversion of hci_req_sync to use it), so it can be safely removed.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>