Remove unused legacy method

When we switched to batched LFS downloads we used a feature flag to
split the code paths.

We cleaned up this flag in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/133078 but we
didn't remove the legacy_download_objects! method introduced in the
original MR
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/122221.

This change removes the method from the codebase.