Remove project bot authorization refresh priority feature flag

What does this MR do and why?

This removes the deprioritize_destroyed_project_bot_user_project_authorizations_refresh feature flag. New behavior is kept - Users::DestroyService schedules the project authorization refresh when a project bot user is destroyed with medium urgency instead of high urgency.

The flag has been enabled globally in production since August 7, 2026, and it was default-enabled for the whole 19.4 milestone.

References

• Rollout issue: [FF] `deprioritize_destroyed_project_bot_user_p... (#607948 - closed)
• Part of https://gitlab.com/groups/gitlab-org/-/work_items/22633+

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