Adjust visibility of private fork in GraphQL project type spec

What does this MR do and why?

Finishing a Community contribution (!112889 (closed)) as per https://about.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows.html#merge-request-coaches. See !112889 (comment 1555127450).

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This MR aims to solve the issue #392331 (closed)

It looks like there is no difference between public and private forks, as a follow up I removed the redundant line and created test data using public_fork and private_fork together .

How to set up and validate locally

bin/rspec spec/graphql/types/project_type_spec.rb

MR acceptance checklist

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