Delete old course units page and widgets

[Atum555]

Closes #1542
Delete old course units page, its route and widgets.

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11%. Comparing base (352dd39) to head (3b42479).

❌ Your project check has failed because the head coverage (11%) is below the target coverage (70%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##           ui/redesign   #1543    +/-   ##
============================================
+ Coverage           11%     11%    +1%     
============================================
  Files              263     261     -2     
  Lines             7741    7600   -141     
============================================
- Hits               827     826     -1     
+ Misses            6914    6774   -140     

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