Remove `dast_ui_redesign` feature flag

What does this MR do and why?

Removes dast_ui_redesign feature flag as a part of #365480 (closed).

Follow-up code cleanup in !101712 (merged). This is done to keep this MR changes minimal.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

This should impact project > security > configuration > dast and project > security > on_demand_scans pages.

MR acceptance checklist

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