Bluetooth: hci_core: cleanup struct hci_dev

Remove unused and set but otherwise unused 'discovery_old_state'
and 'sco_last_tx' members of 'struct hci_dev'. The first one is
a leftover after commit 182ee45d

 ("Bluetooth: hci_sync: Rework
hci_suspend_notifier"); the second one is originated from ancient
2.4.19 and I was unable to find any actual use since that.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>