Remove PlatformAbstractions reference.
Remove PlatformAbstractions reference.

Contributes to https://github.com/dotnet/runtime/issues/3470

aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore