Remove legacy GraphQL resolver

What does this MR do and why?

This MR removes a legacy GraphQL resolver. This class was introduced to allow modifying the meaning of certain values in RunnerStatusEnum while preserving backward compatibility. It can be removed in %17.0 after having being deprecated and made a no-op in %16.0 (legacy_mode was be hard-coded to nil).

Part of #360545 (closed)

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