refactor: delete old unused Altinn header

[wrt95]

Description

• Removing the old unused header component from shared. This is no longer used. See feat: Unique StudioPageHeader component #13365 for full PR with video

Related Issue(s)

• PR itself

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

[wrt95]

Blocked until this PR is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.51%. Comparing base (85c907d) to head (a8a34c6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13745      +/-   ##
==========================================
+ Coverage   94.50%   94.51%   +0.01%     
==========================================
  Files        1620     1605      -15     
  Lines       21713    21536     -177     
  Branches     2569     2542      -27     
==========================================
- Hits        20519    20355     -164     
+ Misses        943      938       -5     
+ Partials      251      243       -8     

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