Remove unused GetMilestonesQuery

What does this MR do and why?

Removes unused files. I believe we migrated from the GraphQL approach to the NDJSON pipeline approach, and this was a leftover. I couldn't find usage of it anywhere.

Nowadays we import group milestones using the BulkImports::Groups::Pipelines::MilestonesPipeline, which uses a ::BulkImports::Common::Extractors::NdjsonExtractor, not a GraphQL one.

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