The `lfs_auto_link_fork_source` feature flag was shipped in GitLab
14.6 via https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75972
and enabled by default in GitLab 14.7.

This feature automatically links LFS objects to a fork if the parent
already has the same object, eliminating the need for the client to
transfer the files again.

Since this has been working well in production for two months,
drop the feature flag.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/348243

Changelog: changed