Remove hashed storage migration codes (First minor release)

What does this MR do and why?

Since GitLab 14.0, there are not any projects using the legacy storage, and it's sure that there won't be any jobs of those workers in the backlog.

This MR removes all related rake tasks, workers, services, and tests that supported the migration to the Hashed Storage in Gitlab 13.4.

The worker classes removal follows the Removing worker classes guideline and this MR is the first minor release.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #351060 (closed)