Remove self_managed_welcome_onboarding feature flag

What does this MR do and why?

Removes the self_managed_welcome_onboarding feature flag and makes its enabled behavior permanent.

This is the FF cleanup for https://gitlab.com/gitlab-org/gitlab/-/work_items/594810. The flag was instance-scoped (no actor) and gated the self-managed admin welcome flow (redirect to /admin/registrations/groups/new + Step 2 profile form).

Changes

• Controllers: Removed Feature.enabled?(:self_managed_welcome_onboarding, :instance) checks from SessionsController#should_redirect_to_sm_onboarding?, Admin::Registrations::GroupsController#verify_available!, and Admin::Registrations::ProfilesController#verify_available!
• Feature flag definition: Deleted config/feature_flags/wip/self_managed_welcome_onboarding.yml
• Specs: Removed stub_feature_flags calls, deleted flag-disabled contexts, and flattened flag-enabled contexts to run unconditionally

No behavior change beyond making the enabled path permanent.

References

• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/594810
• Epic: https://gitlab.com/groups/gitlab-org/-/epics/19488

MR acceptance checklist

• This MR does not introduce behavior changes beyond removing the feature flag
• Tests updated — flag-disabled paths removed, flag-enabled paths flattened
• RuboCop: no offenses
• RSpec: 92 examples, 0 failures