chore: remove transcript editor rollout toggle [LP-1108]

[abhalsod-sonata]

Removes the temporary rollout toggle gating the in-platform transcript editor, per the product
decision to ship the feature without a flag, and brings the fork in line with the upstream
contribution (openedx#3215).

• src/data/api.ts: drop the enableTranscriptEditor waffle-flag default.
• TranscriptMenu.jsx: drop the useWaffleFlags/VideosPageContext imports, the courseId
  context read, the hook call, and the {enableTranscriptEditor && …} wrapper - the
  "Edit transcript" menu item now always renders for videos.
• TranscriptEditor.scss: one trailing-whitespace fix, matching upstream.

Removal only - no new files, no new dependencies, no test changes needed (no fork test
referenced the flag).

Supporting information

• Jira: https://2u-internal.atlassian.net/browse/LP-1108
• Upstream PR this brings the fork in line with: feat: add in-platform transcript editor to the videos page openedx/frontend-app-authoring#3215
• Companion fork PR (backend): chore: remove transcript editor rollout toggle [LP-1108] edx-platform#452
• Original downstream PRs that introduced the toggle: feat: implemented transcript editor #97, fix(transcript-editor): add active cue auto-scroll and responsive modal layout fixes #98

Testing instructions

npm test -- src/files-and-videos

Manual: Course → Videos → open a transcript row's ⋮ menu. "Edit transcript" is present with no
waffle flag configured; the editor opens and saves as before.

Other information

• Deploy effect - user-visible: this turns the transcript editor on for all course teams
  in Studio on deploy. Previously it was hidden unless contentstore.enable_transcript_editor
  was enabled for the course. This is the intended product outcome (LP-1108).
• No ordering dependency on the backend PR: if this deploys first, the removed flag is simply
  ignored; if the backend deploys first, the entry stays hidden until this ships.
• Next parity step (separate PR): port the TypeScript conversion of transcript-editor/
  from upstream feat: add in-platform transcript editor to the videos page openedx/frontend-app-authoring#3215 back to the fork.