kconfig: qconf: do not use 'menu' variable for (QMenuBar *)

I think it is a bit confusing to use 'menu' to hold a QMenuBar pointer.
I want to use 'menu' for a QMenu pointer.

You do not need to use a local variable here. Use menuBar() directly.

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