Remove the dap_onboarding_empty_states feature flag

What does this MR do and why?

Removes the feature flag that controls the DAP empty state panel's visibility.

Also updates all the affected specs as we had originally kept the FF disabled in those due to time constraints.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/587848+

Screenshots or screen recordings

• Refer to the original MR: Implement the AI panel empty state component (!219592 - merged)

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