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>

net/bluetooth/hci_core.c

@@ -3005,8 +3005,6 @@ void hci_adv_monitors_clear(struct hci_dev *hdev)
 		hci_free_adv_monitor(monitor);
 
 	idr_destroy(&hdev->adv_monitors_idr);
-
-	hci_update_background_scan(hdev);
 }
 
 void hci_free_adv_monitor(struct adv_monitor *monitor)