REST: Remove support for project_type filter

What does this MR do and why?

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This MR removes support for the project_type value that was deprecated with Deprecate /groups/:id/runners?type=project_type (!79450 - merged)

REST API: The GET /groups/:id/runners?type=project_type endpoint was deprecated in %14.8 and should be removed in %15.0, since all it did was return an empty collection.

Deprecation issue: REST API: Remove project_type as a valid `type`... (#351466 - closed)

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