Remove SourceBuiltRepoToolsetSdkResolver (our workaround for MSBuild NuGet SDK resolver inflexibility)

[dagood]

Right now, we use it to avoid modifying both the NuGet.Config and global.json to point to a source-built SDK. Ideally we should be able to pass parameters that customize the sources and versions. We need dotnet/msbuild#2914 to do this.

Another alternative is to replace SourceBuiltRepoToolsetSdkResolver with some file replacing in nuget.config and global.json, if SourceBuiltRepoToolsetSdkResolver itself is a maintenance cost. (So far I'm not aware of any.)

/cc @eerhardt @dseefeld @crummel

[MichaelSimons]

[Triage] source-build modifies the global.json to specify the SDK. With the VMR, it would be possible to cloak away the repos global.json and nuget.config then use a common shared VMR version.