Remove gitlab CI artifacts

[kwannoel]

Once Github actions is stable (Probably latest by 30 Sep 2021), Remove the following gitlab ci artifacts:

• [ ] This code in our ci scripts:

  (define-entry-point (check-git-up-to-date)
    (help: "Check that this git checkout is up-to-date with its target branch"
     getopt: [])
    (def up-to-date? (git-up-to-date-with-branch?))
    (printf "Checkout~a up-to-date with branch ~a\n" (if up-to-date? "" " not") (git-origin-branch))
    (silent-exit up-to-date?))
  

  and its use in scripts/ci.ss:

  (run-process/batch ["./unit-tests.ss" "check-git-up-to-date"])
  

  It was only previously necessary because of Gitlab checking the branch-alone and not the "merged" version of MRs Thanks to @AlexKnauth for flagging this.

• [ ] gitlab-ci.yml config.