Remove hooks backup test from backup.rake specs

What does this MR do and why?

#375254 (closed)

This removes a test that was accessing a gitaly managed filesystem to check that a backup had worked correctly. This will not be possible when we start using gitaly cluster by default as it will be impossible to determine where the file is located.

In gitaly#4438 (comment 1112606409) the gitaly team effectively decided that we are okay removing coverage from gitlab-rails as long as it is also covered by gitaly itself.

• Create test https://gitlab.com/gitlab-org/gitaly/-/blob/d897d27c602d80b247af46a4ce672c2cd9b591ba/internal/backup/backup_test.go#L57-69
• Restore test https://gitlab.com/gitlab-org/gitaly/-/blob/d897d27c602d80b247af46a4ce672c2cd9b591ba/internal/backup/backup_test.go#L336-347

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