Comment in code about nuget sdk resolver references unused nuget path

### Steps to reproduce

According to the comment and code here, we load nuget assemblies at runtime and use this environment variable to overload that. However, according to Rainer, that's no longer used and so it appears this exception and the underlying nuget path variable should be removed.
https://github.com/microsoft/msbuild/blob/b4f5792a8fadab87e5199ec54d355c3ba1397a6a/src/Build/BackEnd/Components/SdkResolution/SdkResolverService.cs#L120-L129

### Expected behavior
Remove that exception, the comment, and the NuGetAssemblyPathEnvironmentVariableName definition.

### Actual behavior
A confusing and incorrect error.