W32: Don't unregister parent window of an embedded widget
W32: Don't unregister parent window of an embedded widget

This leads to an assertion failure, because parent window is never registered
in the first place, widget's own GdkWindow is. But that window is unregistered
in a generic fashion by GtkWidget code, so there's nothing for us to do here.

https://bugzilla.gnome.org/show_bug.cgi?id=733766

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