Geo: Delete project_repositories rows where Git repos don't exist

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This is part of a multi-step implementation plan to handle Geo verification/replication of projects without Git repositories to reduce errors caused when Gitaly tries to fetch non-existent repos.

• We want to remove all project_repositories records, where no Git repo exists for the corresponding project.
• We need to find all places where ProjectRepository model is used and if this deletion of records impacts the use.