Bluetooth: MGMT: Remove unused mgmt_*_discovery_complete

mgmt_start_discovery_complete() and mgmt_stop_discovery_complete() last
uses were removed in 2022 by
commit ec2904c2 ("Bluetooth: Remove dead code from hci_request.c")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>