schemas: Remove window shading

Window shading was never really supported in Gnome 3, was never implemented for Wayland and the remaining support was removed from Mutter in version 44.

Drop the remaining references to make it clear that the feature is not supported any more.

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P.S.: I'm not sure this is the correct way to remove such features - feedback very welcome.

Related:

• mutter!2884 (merged)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1830671