This builds on the optimisations in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57482 to further
reduce N+1s during loading source and target project associations.

The N+1 issues happen when a collection of merge requests are serialised
as `API::Github::Entities::PullRequest` entities.

This change also removes duplicate `preload` logic which was made
redundant after 7856804b.