Resolve "High number of statement timeouts in GroupDestroyWorker due to SiteStatistics"

What does this MR do?

• Recalculate SiteStatistics
• Change Group deletion code to prevent triggering after hooks twice (see: https://gitlab.com/gitlab-org/gitlab-ce/issues/50441#note_96535007)

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/50441

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #50441 (closed)