kconfig: menuconfig: remove jump_key::index

You do not need to remember the index of each jump key because you can
count it up after a key is pressed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jesse Taube <Mr.Bossman075@gmail.com>

scripts/kconfig/expr.h

@@ -275,7 +275,6 @@ struct jump_key {
 	struct list_head entries;
 	size_t offset;
 	struct menu *target;
-	int index;
 };
 
 extern struct file *file_list;