Resolve "Resolve minor technical debts of GroupReleasesFinder"

Allen Cookrequested to merge
356131-resolve-minor-technical-debts-of-groupreleasesfinder into master

What does this MR do and why?

This MR resolves several technical debut issues from the previous MR which added [GroupReleasesFinder](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80093)

  • Removing unused code and specs
  • Removing include_subgroups option as it was initially a workaround for poor performance, which has been addressed
  • Refactor GroupReleasesFinder to simplify the preload option

Screenshots or screen recordings

No external changes should be noticeable as this is an internal refactor only

How to set up and validate locally

No external changes should be noticeable as this is an internal refactor only

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #356131 (closed)

Edited by Shinya Maeda

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