kconfig: nconf: remove unneeded default for menu prompt

The rootmenu always has a prompt even if the 'mainmenu' statement is
missing in the top Kconfig file.

conf_parse() calls menu_add_prompt(P_MENU, "Main menu", NULL) in this
case.

So, every 'menu' has a prompt.

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