chore: remove waffle flags for legacy UI

[salman2013]

Summary

Removes all useNew* waffle flag checks that were gating the legacy Studio UI fallbacks. These flags are no longer needed — the new React-based UI pages are now always used.

Changes

• Header.tsx — getOutlineLink() always returns /course/${contextId}; isNewHomePage hardcoded to true; removed useWaffleFlags import
• Breadcrumbs.tsx — getPathToCourseOutlinePage and getPathToCourseUnitPage always resolve to new-UI paths; removed useWaffleFlags and getConfig imports
• AddComponent.tsx — removed useNewPdfEditor flag; PDF blocks always open the new React editor
• ChecklistItemBody.jsx — certificate and outline links always point to new-UI routes; removed useWaffleFlags import

Flags removed

useNewCourseOutlinePage, useNewHomePage, useNewUnitPage, useNewPdfEditor, useNewCertificatesPage

Testing

• Course outline link in header and breadcrumbs navigates to MFE (apps.local.openedx.io:2001)
• Certificate link in course checklist goes to /course/<id>/certificates
• PDF block creation opens the React editor modal inline
• Breadcrumb navigation at section/subsection/unit levels stays within MFE

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