Fix ValueTuple consumption on >= net471 (#215)

* Fix ValueTuple consumption on >= net471

System.ValueTuple is inbox on .NET Framework >= 4.7.1 and therefore any potential redirect for it should be removed. This is necessary as the assembly version in the already shipped packages is higher than what's provided inbox on .NET Framework

* Update src/System.ValueTuple/src/System.ValueTuple.csproj

Co-authored-by: Eric StJohn <ericstj@microsoft.com>

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Co-authored-by: Carlos Sánchez López <1175054+carlossanlop@users.noreply.github.com>
Co-authored-by: Eric StJohn <ericstj@microsoft.com>

Remove PostBuildSign (#173)

PostBuildSigning requires an [additional release and signing step](https://github.com/dotnet/arcade/blob/488a796fd621ef6a81ef08f27e32ae43af1bb96c/Documentation/CorePackages/PostBuildSigning.md) after build.  We aren't doing that for maintenance-packages, so just sign during the build.