add-i: finally retire add.interactive.useBuiltin

add-i: finally retire add.interactive.useBuiltin

The configuration variable stopped doing anything (other than
announcing itself as a variable that does not do anything useful,
when it is used) in Git 2.40.

At this point, it is not even worth giving the warning, which was
meant to be a way to help users notice they are carrying unused
cruft in their configuration files and give them a chance to
clean-up.

Let's remove the warning and documentation for it, and truly stop
paying attention to it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
               ---
 Documentation/config/add.txt |  6 ------
 builtin/add.c                |  6 +-----
 t/t3701-add-interactive.sh   | 15 ---------------
 3 files changed, 1 insertion(+), 26 deletions(-)