mutter does not always destroy virtual monitors, when respective PW stream and DBus interface for stream is detroyed

Affected version: mutter/main, g-s/main

Reproduction steps:

1. Build g-r-d with multimonitor handling (that is with gnome-remote-desktop!186 (merged))
2. Connect with an RDP client, while allowing multimon configurations to be used in the RDP client (when using xfreerdp, use the options /dynamic-resolution /f /multimon for this; when using mstsc, tick the respective multimon checkbox in the display tab).
3. Open in the remote desktop session the display panel in g-c-c (to have the monitor number indicators)
4. Change the display configuration by e.g. removing one monitor on the client side. g-r-d receives a new config via the DISP channel and applies it.
5. Add the monitor again, so that the second monitor is recreated on the mutter side again
6. Remove the monitor again

Observation: You should now have only 1 monitor in the remote desktop session - in theory. In reality, you have now 2 or 3 monitors in your session. This is easily visible, since your current monitor is not the primary anymore (thus, there must be at least another one here) + the monitor count in the top left corner (due to having the display panel open in g-c-c) is now 2 or 3.

It may be the case, that this bug resolves itself by closing the whole remote desktop session. But this is just a guess (and wouldn't solve the original problem anyway).