Remove Usings.cs from samples

[cremor]

The global usings should not be needed any more with #2701

But they now need to be added to the "Below is the equivalent project configuration when not using MSTest.Sdk" sections in the csproj files as <Using Include="Microsoft.VisualStudio.TestTools.UnitTesting" />.

[Jakub Jareš]

Thanks, please see the linked PR, I was not sure if I am getting your request correctly.