builtin.h: remove cmd_tar_tree() declaration

The cmd_tar_tree() function itself was removed in
925ceccf05 (tar-tree: remove deprecated command, 2013-11-10).

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

builtin.h

@@ -225,7 +225,6 @@ int cmd_submodule__helper(int argc, const char **argv, const char *prefix);
 int cmd_switch(int argc, const char **argv, const char *prefix);
 int cmd_symbolic_ref(int argc, const char **argv, const char *prefix);
 int cmd_tag(int argc, const char **argv, const char *prefix);
-int cmd_tar_tree(int argc, const char **argv, const char *prefix);
 int cmd_unpack_file(int argc, const char **argv, const char *prefix);
 int cmd_unpack_objects(int argc, const char **argv, const char *prefix);
 int cmd_update_index(int argc, const char **argv, const char *prefix);