When unlinking installation from organization, delete ProjectGithubRepos

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Problem

Unlinking a GithubAppInstallation from an Organization deletes the related OrganizationGithubAppInstallation record.

That, however, currently in no way affects ProjectGithubRepo records which may link a GithubRepo belonging to the GithubAppInstallation with a project belonging to the Organization.

We should also delete the related ProjectGithubRepo records when the installation is unlinked.