Bluetooth: Notify adv monitor removed event

This notifies management sockets on MGMT_EV_ADV_MONITOR_REMOVED event.

The following test was performed.
- Start two btmgmt consoles, issue a btmgmt advmon-remove command on one
console and observe a MGMT_EV_ADV_MONITOR_REMOVED event on the other.

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>