System value tuple net462 only

[DerAtro]

• Defined System.ValueTuple package reference as conditional for net462 only to reduce package references for other TFMs
• Removed the net46 conditional references to System and Microsoft.CSharp as they are both included by SDK and net46 condition is always false with the current TFMs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.69%. Comparing base (c066b4f) to head (54e6e62).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #574   +/-   ##
=======================================
  Coverage   94.69%   94.69%           
=======================================
  Files           6        6           
  Lines         848      848           
  Branches       92       92           
=======================================
  Hits          803      803           
  Misses         34       34           
  Partials       11       11           

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