Bluetooth: Fix kernel oops triggered by hci_adv_monitors_clear()

This fixes the kernel oops by removing unnecessary background scan
update from hci_adv_monitors_clear() which shouldn't invoke any work
queue.

The following test was performed.
- Run "rmmod btusb" and verify that no kernel oops is triggered.

Signed-off-by: Miao-chen Chou <mcchou@chromium.org>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Reviewed-by: Alain Michaud <alainm@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>