This removes a whole set of GUCs which are no longer used in the
codebase. Also remove relevant documentation.
Reviewed-by: NAshwin Agrawal <aagrawal@pivotal.io>
Reviewed-by: NHeikki Linnakangas <hlinnakangas@pivotal.io>
Reviewed-by: NGeorgios Kokolatos <gkokolatos@pivotal.io>