As a result of removing Razor's VS projects and upgrading all projects to netcoreapp3.0, we no longer need dependency variable for .NET Core 1.x and 2.x and Visual Studio packages.
Changes:
* Remove 'StandardTestTfm'
* Remove variables for .NET Core 1.0, 1.1, 2.0, 2.1, and 2.2
* Remove VSIX variables
* Stop generating 'branding.props' - this hasn't been used in a while and is no longer needed