patchbuiltin.h: remove declaration of cmd_rebase__helper

> 
> Commit 94d4e2fb88 ("rebase -i: move rebase--helper modes to
> rebase--interactive", 2018-07-24) removed the definition of the
> 'cmd_rebase__helper' symbol, but forgot to remove the corresponding
> declaration in the 'builtin.h' header file.
> 
> Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
> ---
> 
> Hi Alban,
> 
> If you need to re-roll your 'ag/rebase-i-in-c' branch, could you
> please squash this into the relevant patch (commit 94d4e2fb88).
> 
> Thanks!
> 
> ATB,
> Ramsay Jones
> 
>  builtin.h | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/builtin.h b/builtin.h
> index aac8f5f340..6538932e99 100644
> --- a/builtin.h
> +++ b/builtin.h
> @@ -206,7 +206,6 @@ extern int cmd_range_diff(int argc, const char **argv, const char *prefix);
>  extern int cmd_read_tree(int argc, const char **argv, const char *prefix);
>  extern int cmd_rebase(int argc, const char **argv, const char *prefix);
>  extern int cmd_rebase__interactive(int argc, const char **argv, const char *prefix);
> -extern int cmd_rebase__helper(int argc, const char **argv, const char *prefix);
>  extern int cmd_receive_pack(int argc, const char **argv, const char *prefix);
>  extern int cmd_reflog(int argc, const char **argv, const char *prefix);
>  extern int cmd_remote(int argc, const char **argv, const char *prefix);
>