Stop deleting the db directory in backup rake specs

What does this MR do and why?

Fixes #462975 (closed)

This code was inadvertently deleting relative to rails root rather than the backup directory. Since this code was never doing what it was supposed to, it is safe just to remove it.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Run the backup rake specs:

   $ bundle exec rspec spec/tasks/gitlab/backup_rake_spec.rb

2. Check that the git worktree is clean:

   $ git status
   ...
   nothing to commit, working tree clean

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