Bluetooth: Delete unused hci_req_stop_discovery()

hci_req_stop_discovery has been deprecated in favor of
hci_stop_discovery_sync() as part of transition to hci_sync.c

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