adv_monitor: Implement UnregisterMonitor()

adv_monitor: Implement UnregisterMonitor()

This implements the UnregisterMonitor() method handler of ADV monitor
manager interface.

The following tests were performed.
- Issue a UnregisterMonitor() call with a nonexistent path and expect
org.bluez.Error.DoesNotExist as the return.
- Issue a UnregisterMonitor() call with a invalid path and expect
org.bluez.Error.InvalidArguments as the return.
- Issue RegisterMonitor() with a path, issue UnregisterMonitor() and
expect a successful method call return.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>