Trim dependencies of netcore packages

[enricosada]

The pr #1225 trim dependencies in project.json but that's used only to restore locally the packages to a directory (like old packages.config )

The nuspec https://github.com/Microsoft/visualfsharp/blob/master/src/fsharp/fsharp.core.netcore.nuget/Microsoft.FSharp.Core.netcore.nuspec should be updated too