[DO NOT MERGE] feat!: remove custom header settings

[jsnwesson]

This PR aims to remove the custom header settings for Learner Dashboard. Most of these features already exist as the default experience in frontend-component-header, minus the Programs/Discover New tabs, the Support link, and other 2U-specific features.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (9ef5840) to head (3663efb).
⚠️ Report is 104 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
- Coverage   97.52%   97.47%   -0.05%     
==========================================
  Files         152      149       -3     
  Lines        1332     1307      -25     
  Branches      229      221       -8     
==========================================
- Hits         1299     1274      -25     
  Misses         32       32              
  Partials        1        1              

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[jsnwesson]

Todo: I think there are some messages and track events that can be deleted as well.