Revert "Test | Expose internal types to ManualTests"

[mdaigle]

Reverts #3239

This change was causing confusion because it's only a partial solution and only works in Visual Studio.
Visual Studio build uses implementation assemblies, which contain internal types.
MSBuild uses the reference assemblies, which don't contain internal types.

We can reapply this change when we've updated our reference assemblies to include internal types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.03%. Comparing base (61e2317) to head (2383f9f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3268      +/-   ##
==========================================
+ Coverage   72.81%   73.03%   +0.22%     
==========================================
  Files         297      296       -1     
  Lines       59661    57254    -2407     
==========================================
- Hits        43440    41817    -1623     
+ Misses      16221    15437     -784     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.30% <ø> (+0.11%)	⬆️
netfx	71.43% <ø> (-0.06%)	⬇️

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