nconfig: Avoid Wunused-but-set warning

I am seeing Wunused-but-set warning while make nconfig.  Looks like
active_menu is not used. Removing it fixes the warning.

Signed-off-by: Raghavendra D Prabhu <rprabhu@wnohang.net>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>