Remove superseded SSH certificates protocol option
What does this MR do and why?
It was introduced in Enforce auth via SSH certificates for Git access (!132653 - merged) as a solution to Enforce authentication via SSH certificates for... (#421917 - closed).
However, this approach wasn't flexible enough to cover all use cases, so Enforce SSH Certificates via Settings (!136498 - merged) was introduced.
Since the old approach is superseded and wasn't enabled for all customers, we can go ahead and remove it