Remove merge_request specific code from Relation Factory

This MR tries to remove :merge_request specific code introduced in 17844 from RelationFactory. We are still relying on EXISTING_OBJECT_CHECK. I just extended GroupProjectObjectBuilder added some logic to :where_clause_for_klass, so we can use it in more general way.

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

We need to make sure that when we have use case where ci_pipeline is created during merge request event, that merge request is still imported correctly.