Clean up FF expiring_pats_30d_60d_notifications

What does this MR do and why?

Clean up expiring_pats_30d_60d_notifications FF

Removes the feature flag for 30-day and 60-day access token expiry notification emails, making the feature generally available.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Feature issue: #464040 (closed)
• Release post: gitlab-com/www-gitlab-com!136008 (merged)
• Implementation MR: !166683 (merged)

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. Enable the feature: Feature.enable(:expiring_pats_30d_60d_notifications)
2. Go to your account Personal Access Tokens: https://gdk.test:3443/-/user_settings/personal_access_tokens
3. Create 3 access tokens:
   1. expiring in 5 days
   2. expiring in 20 days
   3. expiring in 55 days
4. Go to access tokens for a project you own (with Ultimate plan if emulating SaaS): https://gdk.test:3443/flightjs/Flight/-/settings/access_tokens
5. Create 3 access tokens:
   1. expiring in 5 days
   2. expiring in 20 days
   3. expiring in 55 days
6. Go to access tokens for a group you own (with Ultimate plan if emulating SaaS): https://gdk.test:3443/flightjs/-/settings/access_tokens
   1. expiring in 5 days
   2. expiring in 20 days
   3. expiring in 55 days
7. Run the worker from the Rails console: PersonalAccessTokens::ExpiringWorker.new.perform or enqueue from https://gdk.test:3443/admin/sidekiq/cron
8. Go to letter_opener and check for notifications about the expiring tokens: https://gdk.test:3443/rails/letter_opener
9. If you don't see the emails, you can validate that the mailers are enqueued in Sidekiq at https://gdk.test:3443/admin/sidekiq/queues/mailers

Related to #464040 (closed)