feat: remove ENABLE_COURSE_OUTLINE_NEW_DESIGN and ENABLE_UNIT_PAGE_NEW_DESIGN flags

[rpenido]

Description

Describe what this pull request changes, and why. Include implications for people using this change.
Design decisions and their rationales should be documented in the repo (docstring / ADR), per
OEP-19, and can be linked here.

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Best Practices Checklist

We're trying to move away from some deprecated patterns in this codebase. Please
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• Any new files are using TypeScript (.ts, .tsx).
• Avoid propTypes and defaultProps in any new or modified code.
• Tests should use the helpers in src/testUtils.tsx (specifically initializeMocks)
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• Use React Query to load data from REST APIs. See any apiHooks.ts in this repo for examples.
• All new i18n messages in messages.ts files have a description for translators to use.
• Avoid using ../ in import paths. To import from parent folders, use @src, e.g. import { initializeMocks } from '@src/testUtils'; instead of from '../../../../testUtils'

[openedx-webhooks]

Thanks for the pull request, @rpenido!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.38%. Comparing base (f5d874a) to head (896b190).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2992      +/-   ##
==========================================
- Coverage   95.57%   95.38%   -0.20%     
==========================================
  Files        1366     1364       -2     
  Lines       31507    31360     -147     
  Branches     6885     7065     +180     
==========================================
- Hits        30113    29912     -201     
- Misses       1344     1397      +53     
- Partials       50       51       +1     

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