Bluetooth: Remove unnecessary call to hci_update_background_scan

The hci_conn_params_clear_all() function is only called from
hci_unregister_dev() at which point it's completely futile to try to
do any LE scanning updates. Simply remove this unnecessary function
call. At the same time we can make the function static since it's only
accessed from within the same c-file.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>