kconfig: squash menu_has_help() and menu_get_help()

menu_has_help() and menu_get_help() functions are only used within
menu_get_ext_help().

Squash them into menu_get_ext_help(). It revealed the if-conditional
in menu_get_help() was unneeded, as menu_has_help() has already checked
that menu->help is not NULL.

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