Remove SqlCommand code paths for context connections

[edwardneal]

Contributes to #2838. PR 2/7.

This doesn't need to be merged before 6.0. It removes the core SqlDataReaderSmi type, and the code paths in SqlCommand.ExecuteNonQuery and SqlCommand.ExecuteReader which reference it. There are also a handful of Stream and TextReader derivatives which are only used in those contexts.

One point of note is in SqlDataRecord. InOutOfProcHelper.InProc will always be false, so I've trimmed the dead code in the if condition. As a result, _recordContext will always be null, and null will always propagate to the context parameter in ValueUtilsSmi. I plan to clean this up in the next PR.

If this is too large to review, I can partially revert some of these removals.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.52%. Comparing base (4a9b070) to head (fa01093).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2996      +/-   ##
==========================================
+ Coverage   72.45%   73.52%   +1.06%     
==========================================
  Files         288      283       -5     
  Lines       59498    58652     -846     
==========================================
+ Hits        43110    43122      +12     
+ Misses      16388    15530     -858     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.42% <ø> (+0.05%)	⬆️
netfx	72.43% <100.00%> (+1.49%)	⬆️

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