Cleanup W3CTraceContextTests

[Kielek]

Fixes #5832 (review) - part 3.

Changes

Cleanup W3CTraceContextTests after dropping support for .NET6

• semi automatic cleanup of suggestions in this file.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.30%. Comparing base (6250307) to head (037d27f).
Report is 327 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5858      +/-   ##
==========================================
+ Coverage   83.38%   86.30%   +2.91%     
==========================================
  Files         297      257      -40     
  Lines       12531    11185    -1346     
==========================================
- Hits        10449     9653     -796     
+ Misses       2082     1532     -550     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.28% <ø> (?)
unittests-Project-Stable	86.12% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

see 225 files with indirect coverage changes

[CodeBlanch]

LGTM