Remove System.ValueTuple dependency from net471 nuget package

[glopesdev]

I believe it is not necessary to include System.ValueTuple as a dependency when targeting net471. Ideally this dependency would be included only for netstandard2.0 to prevent binding redirect conflicts (see dotnet/standard#543).

[glopesdev]

Interestingly, removing System.ValueTuple entirely could be another option, as the project seems to build fine without it, even on netstandard2.0.

[glopesdev]

In fact, it seems we could go further and target netstandard2.0 only. It comes built-in from .NET 4.7.1 onwards: dotnet/standard#514