[tests] Remove unused CustomTextMapPropagator

[Kielek]

Changes

Remove unused CustomTextMapPropagator

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 86.19%. Comparing base (6250307) to head (7b3643c).
Report is 277 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5743      +/-   ##
==========================================
+ Coverage   83.38%   86.19%   +2.81%     
==========================================
  Files         297      254      -43     
  Lines       12531    11057    -1474     
==========================================
- Hits        10449     9531     -918     
+ Misses       2082     1526     -556     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.10% <ø> (?)
unittests-Project-Stable	86.15% <ø> (?)

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see 196 files with indirect coverage changes