dir-iterator: drop unused `DIR_ITERATOR_FOLLOW_SYMLINKS`

The `FOLLOW_SYMLINKS` flag was added to the dir-iterator API in
fa1da7d2ee (dir-iterator: add flags parameter to dir_iterator_begin,
2019-07-10) in order to follow symbolic links while traversing through a
directory.

`FOLLOW_SYMLINKS` gained its first caller in ff7ccc8c9a (clone: use
dir-iterator to avoid explicit dir traversal, 2019-07-10), but it was
subsequently removed in 6f054f9fb3 (builtin/clone.c: disallow `--local`
clones with symlinks, 2022-07-28).

Since then, we've held on to the code for `DIR_ITERATOR_FOLLOW_SYMLINKS`
in the name of making minimally invasive changes during a security
embargo.

In fact, we even changed the dir-iterator API in bffc762f87
(dir-iterator: prevent top-level symlinks without FOLLOW_SYMLINKS,
2023-01-24) without having any non-test callers of that flag.

Now that we're past those security embargo(s), let's finalize our
cleanup of the `DIR_ITERATOR_FOLLOW_SYMLINKS` code and remove its
implementation since there are no remaining callers.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/dir-iterator.c b/dir-iterator.c
index 3764dd8..cedd304 100644
--- a/dir-iterator.c
+++ b/dir-iterator.c
@@ -112,10 +112,7 @@ static int prepare_next_entry_data(struct dir_iterator_int *iter,
 	iter->base.basename = iter->base.path.buf +
 			      iter->levels[iter->levels_nr - 1].prefix_len;
 
-	if (iter->flags & DIR_ITERATOR_FOLLOW_SYMLINKS)
-		err = stat(iter->base.path.buf, &iter->base.st);
-	else
-		err = lstat(iter->base.path.buf, &iter->base.st);
+	err = lstat(iter->base.path.buf, &iter->base.st);
 
 	saved_errno = errno;
 	if (err && errno != ENOENT)
@@ -213,13 +210,10 @@ struct dir_iterator *dir_iterator_begin(const char *path, unsigned int flags)
 	iter->flags = flags;
 
 	/*
-	 * Note: stat/lstat already checks for NULL or empty strings and
+	 * Note: lstat already checks for NULL or empty strings and
 	 * nonexistent paths.
 	 */
-	if (iter->flags & DIR_ITERATOR_FOLLOW_SYMLINKS)
-		err = stat(iter->base.path.buf, &iter->base.st);
-	else
-		err = lstat(iter->base.path.buf, &iter->base.st);
+	err = lstat(iter->base.path.buf, &iter->base.st);
 
 	if (err < 0) {
 		saved_errno = errno;