Remove placeholder configs from restore

[ViktorHofer]

Fixes the restore of projects with placeholder configs in them:

C:\git\runtime4\src\libraries\System.ComponentModel.Composition\ref>dotnet restore /bl
C:\Program Files\dotnet\sdk\3.1.200-preview-014850\MSBuild.dll -nologo -distributedlogger:Microsoft.DotNet.Tools.MSBuild.MSBuildLogger,C:\Program Files\dotnet\sdk\3.1.200-preview-014850\dotnet.dll*Microsoft.DotNet.Tools.MSBuild.MSBuildForwardingLogger,C:\Program Files\dotnet\sdk\3.1.200-preview-014850\dotnet.dll -maxcpucount -target:Restore -verbosity:m /bl .\System.ComponentModel.Composition.csproj
C:\Program Files\dotnet\sdk\3.1.200-preview-014850\NuGet.targets(123,5): error : Invalid restore input. Invalid target framework 'unsupported'. Input files: C:\git\runtime4\src\libraries\System.ComponentModel.Composition\ref\System.ComponentModel.Composition.csproj. [C:\git\runtime4\src\libraries\System.ComponentModel.Composition\ref\System.ComponentModel.Composition.csproj]