packaging: setup: avoid errors in uninstall group

previously if the uninstall group list was specified in
an answer file omitting the trailing ',' cleanup
failed removing both the latest 2 groups.

Change-Id: Ie23de94a31ca393e63a42dfb5b6e383dad3f5983
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>