Remove the unused, EE-only mv_storage gitlab_projects command

What does this MR do?

Removes some dead EE-only code in lib/gitlab/shell.rb

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This code is unused. Removing it allows CE and EE to be closer together. It also clarifies the situation with respect to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15853

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/41016