Bluetooth: Remove dead code from hci_request.c

The discov_update work queue is no longer used as a result
of the hci_sync rework.

The __hci_req_hci_power_on() function is no longer referenced in the
code as a result of the hci_sync rework.

Signed-off-by: Brian Gix <brian.gix@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>