rebase: remove unused function reschedule_last_action

The only caller of reschedule_last_action was removed by ef64bb328d
(rebase: strip unused code in git-rebase--preserve-merges.sh,
2018-05-28); remove this unused shell function as well.

Signed-off-by: René Scharfe <l.s.r@web.de>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--preserve-merges.sh

@@ -193,16 +193,6 @@ mark_action_done () {
 	fi
 }
 
-# Put the last action marked done at the beginning of the todo list
-# again. If there has not been an action marked done yet, leave the list of
-# items on the todo list unchanged.
-reschedule_last_action () {
-	tail -n 1 "$done" | cat - "$todo" >"$todo".new
-	sed -e \$d <"$done" >"$done".new
-	mv -f "$todo".new "$todo"
-	mv -f "$done".new "$done"
-}
-
 append_todo_help () {
 	gettext "
 Commands: