Revert "Merge branch '442284-mhamda-bulk-assign-service' into 'master'"

What does this MR do and why?

We previously relocated this library, which was exclusively utilized in bulk assign rake tasks, to a service to incorporate it into APIs.

However, during testing, it became evident that utilizing the creation service within API calls would result in significant performance issues. For further information, please refer to https://gitlab.com/gitlab-org/gitlab/-/work_items/443497#note_1792855852.

Consequently, we are reverting this change to reinstate the library and address the service specifically for Gitlab.com in !146104 (merged).

In this merge request, we are relocating this library back for its usage in SM rake tasks to avoid missues. Until we implement a similar approach for SM, we should consider deprecating both the rake task and the library.

Deprecation Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/444147