Remove GenerateInternalsVisibleTo.targets

[tmat]

• Is this issue blocking (yes/no): no
• Is this issue causing unreasonable pain (yes/no): no

Remove https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Arcade.Sdk/tools/GenerateInternalsVisibleTo.targets once we move repos to .NET SDK that has this functionality built-in (see dotnet/sdk#3439).