Stop needing SSH keys for Geo primaries

What does this MR do?

Removes the requirement for a Geo primary to have an SSH keypair,

This MR also relaxes rake gitlab:check so it allows secondaries to have an SSH keypair. We currently rely on this mechanism to sync repositories and can't remove it... yet.

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

Why was this MR needed?

The keys are used by Geo secondaries to sync from Geo primaries. There's no need for primaries to pull keys from secondaries.

We're moving to a private-keys-in-database model for %10.1 and this is a partial step towards that.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3340 (closed)