Remove hardcoded PackageVersion in Platforms pkg (#48596)

Mapping the package to `$(ProductVersion)` would be wrong in servicing cases. Hence just removing the hardcoded value entirely.

src/libraries/Microsoft.NETCore.Platforms/pkg/Microsoft.NETCore.Platforms.pkgproj

 <Project DefaultTargets="Build">
   <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.props))" />
   <PropertyGroup>
-    <PackageVersion>$(ProductVersion)</PackageVersion>
     <SkipValidatePackage>true</SkipValidatePackage>
     <!-- We don't need to harvest the stable packages to build this -->
     <HarvestStablePackage>false</HarvestStablePackage>