kconfig: remove string expansion for mainmenu after yyparse()

Now that environments are expanded in the lexer, conf_parse() does
not need to expand them explicitly.

The hack introduced by commit 0724a7c3 ("kconfig: Don't leak
main menus during parsing") can go away.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>