Bluetooth: Remove stop discovery handling from hci_event.c

Since all mgmt stop discovery command complete events are now handled
in stop_discovery_complete callback in mgmt.c, we can remove this
handling from hci_event.c.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>