Code Health | Remove unused/unreferenced files

[edwardneal]

This is largely cleanup from earlier PRs - four files are no longer used:

• SqlTypeWorkarounds.netcore.cs: no longer referenced as a result of the removal of .NET 6.0 support
• ApplicationProtocolNegotiationExt.netcore.cs: see above
• ApplicationProtocolNegotiationStatus.netcore.cs: see above
• SqlFileStream.cs: this is the netfx version. No longer referenced in the csproj, it was just left behind when SqlFileStream was merged

Not essential for preview3, although I don't see it having any impact if included (beyond review time!)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.47%. Comparing base (5744b51) to head (9c64f17).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3008      +/-   ##
==========================================
+ Coverage   72.45%   72.47%   +0.02%     
==========================================
  Files         288      287       -1     
  Lines       59493    59474      -19     
==========================================
+ Hits        43104    43106       +2     
+ Misses      16389    16368      -21     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.41% <ø> (+0.03%)	⬆️
netfx	70.91% <ø> (+0.01%)	⬆️

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