Remove unnecessary Ci::Trace#archived_trace_exists? method

What does this MR do and why?

This MR removes a method that we've effectively had aliased to another existing method, and makes that method public so it can be used throughout the rest of the app. It's a naming change we've wanted to make anyway #343796 (comment 941391666), and we might as well do it now since we have a community contribution !86990 (merged) making the other renaming and we're right at the end of the development milestone for %15.0.

MR acceptance checklist

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