[EE] update net-ssh 4 -> 5, remove rbnacl, rbnacl-libsodium (not needed)

EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19412.

What does this MR do?

Update net-ssh to latest upstream version. This also removes rbnacl dependency, as discussed at !17840 (merged)

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?