kconfig: qconf: do not show checkbox icon for choice

When you select "Show All Options" or "Show Prompt Options", choice
entries display a check box icon, but this has no point because
choice is always y since commit fde19251 ("kconfig: remove tristate
choice support").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>