This adds the request handler of MGMT_OP_REMOVE_ADV_MONITOR command.
Note that the controller-based monitoring is not yet in place. This
removes the internal monitor(s) without sending HCI traffic, so the
request returns immediately.

The following test was performed.
- Issue btmgmt advmon-remove with valid and invalid handles.
Signed-off-by: Miao-chen Chou <mcchou@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>