kconfig: omit --oldaskconfig option for 'make config'

scripts/kconfig/conf.c line 39 defines the default of input_mode as
oldaskconfig. Hence, 'make config' works in the same way even without
the --oldaskconfig option given. Note this in the help message.

This will be helpful to unify build rules in Makefile in the next
commit.

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