rebase -i: remove unused modes and functions

rebase -i: remove unused modes and functions

This removes the modes `--skip-unnecessary-picks`, `--append-todo-help`,
and `--checkout-onto` from rebase--helper.c, the functions of
git-rebase--interactive.sh that were rendered useless by the rewrite of
complete_action(), and append_todo_help_to_file() from
rebase-interactive.c.

skip_unnecessary_picks() and checkout_onto() becomes static, as they are
only used inside of the sequencer.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>