kconfig: remove dead code in conf_askvalue()

conf_askvalue() is only called for oldconfig, syncconfig, and
oldaskconfig. If it is called for other cases, it is a bug.

So, the code after the switch statement is unreachable.

Remove the dead code, and clean up the switch statement.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>