csd: Drop the GTK_FRAME_EXTENTS requirement
csd: Drop the GTK_FRAME_EXTENTS requirement

We are getting bug reports from people who are irritated that
dialogs now have 'double headers' under any wm but gnome-shell.

As an example, xfwm4 seems to do ok with csd windows, and
on balance it seems better to have some invisible border issues
than to have double headers.

https://bugzilla.gnome.org/show_bug.cgi?id=727414

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