x11: Drop support for _NET_WM_FRAME_OVERLAP

The main idea behind _NET_WM_FRAME_OVERLAP is to extend the borders of
the server-side decoration so the application can draw on top of it. It
was inspired by similar feature in Windows.

However, _NET_WM_FRAME_OVERLAP is basically unused. Neither GTK nor Qt
support it and I have never seen any application that uses it.

At the moment, kwin is the only compositing window manager that supports
_NET_WM_FRAME_OVERLAP. Neither mutter nor compiz nor compton and so on
support it.

Since _NET_WM_FRAME_OVERLAP is practically unused, there's no point for
keeping supporting it.

This change shouldn't affect any existing app as _NET_WM_FRAME_OVERLAP
atom is not listed in _NET_SUPPORTED.