Remove support for legacy tar.gz pages artifacts

What does this MR do?

Remove support for legacy artifacts.tar.gz uploads in the gitlab-pages deploy code.

Tar.gz was only used by the CI runner during a brief window around GitLab 8.1 (October 2015).

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

Why was this MR needed?

Deleting unused code is good.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?