t2400: clean up '"add" worktree with lock' test

- remove unneeded `git rev-parse` which must have come from a copy-paste
  of another test
- unlock the worktree with test_when_finished

Signed-off-by: Stephen Manz <smanz@alum.mit.edu>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2400-worktree-add.sh

@@ -67,8 +67,8 @@ test_expect_success '"add" worktree' '
 '
 
 test_expect_success '"add" worktree with lock' '
-	git rev-parse HEAD >expect &&
 	git worktree add --detach --lock here-with-lock main &&
+	test_when_finished "git worktree unlock here-with-lock || :" &&
 	test -f .git/worktrees/here-with-lock/locked
 '