sequencer: remove duplicate rollback_lock_file() call

Commit 2b6ad0f ("rebase --rebase-merges: add support for octopus
merges", 2017-12-21) introduced a case where rollback_lock_file() was
unconditionally called twice in a row with no intervening commands.
Remove the duplicate.

Signed-off-by: Elijah Newren <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

sequencer.c

@@ -3572,7 +3572,6 @@ static int do_merge(struct repository *r,
 		unlink(git_path_cherry_pick_head(r));
 		rollback_lock_file(&lock);
 
-		rollback_lock_file(&lock);
 		ret = run_command(&cmd);
 
 		/* force re-reading of the cache */