Improve Ruby rspec test spec/graphql/types/project_type_spec.rb

The following discussion from !112167 (merged) should be addressed:

• @mikolaj_wawrzyniak started a discussion:

  nitpick (non-blocking): @leetickett-gitlab It looks that there is no difference between public and private forks, as a follow up I would suggest to either make private really private, or remove redundant line

      let_it_be(:fork_public) { fork_project(project, nil, { repository: true }) }
      let_it_be(:fork_private) { fork_project(project, nil, { repository: true }) }