Bluetooth: hci_core, hci_sync: cleanup struct discovery_state

After commit 78db544b ("Bluetooth: hci_core: Remove le_restart_scan
work"), 'scan_start' and 'scan_duration' of 'struct discovery_state'
are still initialized but actually unused. So remove the aforementioned
fields and adjust 'hci_discovery_filter_clear()' and 'le_scan_disable()'
accordingly. Compile tested only.

Fixes: 78db544b

 ("Bluetooth: hci_core: Remove le_restart_scan work")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>