Remove xunit.runner.visualstudio from RDotNet.Tests project.

Is not needed here and causes test command to fail.
This causes warning:
warning NU1701: Package 'xunit.runner.visualstudio 2.3.1' was restored using
'.NETFramework,Version=v4.6.1' instead of the project target framework '.NETStandard,Version=v2.0'.
This package may not be fully compatible with your project.

and error:
Catastrophic failure: System.TypeLoadException: Could not load type 'System.Runtime.Remoting.Channels.IChannel'
from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
   at Xunit.RemotingUtility.CleanUpRegisteredChannels()
   at Xunit.Runner.VisualStudio.VsTestRunner.RunTests(IRunContext runContext, IFrameworkHandle frameworkHandle,
   LoggerHelper logger, TestPlatformContext testPlatformContext, Func`1 getRunInfos)
   in C:\Dev\xunit\xunit\src\xunit.runner.visualstudio\VsTestRunner.cs:line 395