Remove support for lxdialog --checklist

The checklist lxdialog functionality is not used by menuconfig
(only the radiolist variant is used) and supporting it would
significantly complicate the forthcoming liblxdialog API.
Signed-off-by: NPetr Baudis <pasky@suse.cz>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>