Because serde issues #1904 and #2371 are fixed, remove workarounds for them from tests

[Mingun]

Related issues:

• Deserialize failed to derive on enums with tuple variant and flatten struct variant serde-rs/serde#1904
• "undeclared lifetime" when derive Deserialize for an enum with both flatten and 'static fields serde-rs/serde#2371

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.10%. Comparing base (39b5905) to head (5d4a04d).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #821      +/-   ##
==========================================
+ Coverage   60.08%   60.10%   +0.01%     
==========================================
  Files          41       41              
  Lines       15975    15975              
==========================================
+ Hits         9599     9602       +3     
+ Misses       6376     6373       -3     

Flag	Coverage Δ
unittests	60.10% <ø> (+0.01%)	⬆️

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