add API: remove run_add_interactive() wrapper function

add API: remove run_add_interactive() wrapper function

Now that the Perl "git-add--interactive" has gone away in the
preceding commit we don't need to pass along our desire for a mode as
a string, and can instead directly use the "enum add_p_mode", see
d2a233cb8b9 (built-in add -p: prepare for patch modes other than
"stage", 2019-12-21) for its introduction.

As a result of that the run_add_interactive() function would become a
trivial wrapper which would only run run_add_i() if a 0 (or now,
"NULL") "patch_mode" was provided. Let's instead remove it, and have
the one callsite that wanted the "NULL" case (interactive_add())
handle it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>