Remove Gitaly clock sync check readiness spec

What does this MR do and why?

The Gitaly readiness check spec mocks a clock sync readiness failure. Since the readiness RPC in Praefect will no longer perform the clock sync check, this change updates the test to reference a generic readiness check failure.

Screenshots or screen recordings

N/A

How to set up and validate locally

bundle exec rspec spec/lib/gitlab/health_checks/gitaly_check_spec.rb -e "#readiness Gitaly server is not ready"

MR acceptance checklist

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