Remove the `scrum_projects` feature flag and the transitional code branches that were only needed while the old and new Backlogs implementations coexisted.

Begin of the insertionExamples of code that should disappear as part of this work include old taskEnd of the insertion Begin of the deletion This includes removing the feature registration itself, deleting dead conditional paths,End of the deletion and Begin of the insertionimpediment handling paths that are no longer needed onceEnd of the insertion Begin of the deletion makingEnd of the deletion the sprint-based Begin of the deletion BacklogsEnd of the deletion implementation Begin of the insertionis permanent, such as `RbTasksController`, `RbImpedimentsController`, and their associated views, routes, and dead UI code.End of the insertion Begin of the deletion the permanent behaviour.End of the deletion

Begin of the insertionThis also includes removing legacy Backlogs frontend codeEnd of the insertion Begin of the deletion Specific controllersEnd of the deletion that Begin of the insertiononly existed for the old implementation, including obsolete jQuery-based behaviour and related plugin usage where those paths become unreachable or unnecessary.

This ticket supersedes <mention class="mention" data-id="68795" data-type="work_package" data-text="##68795">##68795</mention>&nbsp;End of the insertion Begin of the deletion may&nbsp;End of the deletion

_This ticket should only cover code that exists because of the feature flag or the temporary coexistence of the old and new paths. Broader route consolidation, naming cleanup, style entrypoint cleanup, and final sprint model replacement should remain in their respective child tickets of <mention class="mention" data-id="73233" data-type="work_package" data-text="#73233">#73233</mention> ._