This helps to prevent an obscure problem that we've run into recently.
If metacity leaves _NET_SUPPORTING_WM_CHECK behind (a), and another app manages to reuse that window id (b) before another wm has taken over (c), bad things happen. GTK+ apps that are started between (a) and (c) won't know about the window manager started in (c) until the app started in (b) dies, since they keep using the value of _NET_SUPPORTING_WM_CHECK they got initially.

The attached patch unsets _NET_SUPPORTING_WM_CHECK when metacity stops managing a screen. Note that this is only best-effort, since the property will still be left behind when metacity is just killed.