Resolve "Remove Feature Flag use_pipeline_wizard_for_pages"

What does this MR do and why?

This MR removes the use_pipeline_wizard_for_pages Feature Flag after it's been enabled by default for several months now.

Screenshots or screen recordings

No UI change.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #430346 (closed)