Remove the personal_homepage feature flag

What does this MR do and why?

The personal_homepage feature flag was type: beta, default_enabled: true, and has been enabled on GitLab.com, GitLab Self-Managed, and GitLab Dedicated since GitLab 18.5. This MR removes the flag, keeping the flag-enabled code path and deleting the disabled path. There is no user-visible behavior change on GitLab.com or on any instance that had not explicitly disabled the flag.

References

• Tracking issue: https://gitlab.com/gitlab-org/gitlab/-/issues/577448
• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/561388
• Feature issue: #546151 (closed)
• Introduced by: !192774 (merged)

Screenshots or screen recordings

No screenshots. The flag was already enabled by default, so the rendered UI is unchanged.

How to set up and validate locally

1. Sign in to the GDK as a regular user.
2. Visit the instance root and confirm the personal homepage renders.
3. Visit /dashboard/home and confirm it renders instead of returning a 404.
4. Check that Home appears in the "Your work" sidebar.
5. Open user preferences and confirm Homepage is still offered as a homepage choice.
6. Confirm git grep personal_homepage returns no Feature.enabled? call sites and no flag definition file.

MR acceptance checklist

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