Bluetooth: Remove HCI_POWER_OFF_TIMEOUT

With commit cf75ad8b ("Bluetooth: hci_sync: Convert MGMT_SET_POWERED"),
the power off sequence got refactored so that this timeout was no longer
necessary, let's remove the leftover define from the header too.

Fixes: cf75ad8b ("Bluetooth: hci_sync: Convert MGMT_SET_POWERED")
Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>