add: remove "add.interactive.useBuiltin" & Perl "git add--interactive"

Since [1] first released with Git v2.37.0 the built-in version of "add
-i" has been the default. That built-in implementation was added in
[2], first released with Git v2.25.0.

At this point enough time has passed to allow for finding any
remaining bugs in this new implementation, so let's remove the
fallback code.

As with similar migrations for "stash"[3] and "rebase"[4] we're
keeping a mention of "add.interactive.useBuiltin" in the
documentation, but adding a warning() to notify any outstanding users
that the built-in is now the default. As with [5] and [6] we should
follow-up in the future and eventually remove that warning.

1. 0527ccb1b55 (add -i: default to the built-in implementation,
   2021-11-30)
2. f83dff60a78 (Start to implement a built-in version of `git add
   --interactive`, 2019-11-13)
3. 8a2cd3f5123 (stash: remove the stash.useBuiltin setting,
   2020-03-03)
4. d03ebd411c6 (rebase: remove the rebase.useBuiltin setting,
   2019-03-18)
5. deeaf5ee077 (stash: remove documentation for `stash.useBuiltin`,
   2022-01-27)
6. 9bcde4d5314 (rebase: remove transitory rebase.useBuiltin setting &
   env, 2021-03-23)

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

1 files changed, 4 insertions, 3 deletions

diff --git a/Documentation/config/add.txt b/Documentation/config/add.txt
index 3e859f3419..e0354ceaed 100644
--- a/Documentation/config/add.txt
+++ b/Documentation/config/add.txt
@@ -7,6 +7,7 @@ add.ignore-errors (deprecated)::
 	variables.
 
 add.interactive.useBuiltin::
-	Set to `false` to fall back to the original Perl implementation of
-	the interactive version of linkgit:git-add[1] instead of the built-in
-	version. Is `true` by default.
+	Unused configuration variable. Used in Git versions v2.25.0 to
+	v2.36.0 to enable the built-in version of linkgit:git-add[1]'s
+	interactive mode, which then became the default in Git
+	versions v2.37.0 to v2.39.0.