Remove System.Diagnostics.StackTrace from debugger install (dotnet#620)

The debugger was installing System.Diagnostics.StackTrace which it doesn't
appear to actually need and, in the dev install scenarios at least, was
causing a problem with pulling in the wrong version of
System.Reflection.Metadata. This removes it so the the dev and shipping
code pulls in the same set of assemblies.

src/coreclr-debug/install.ts

@@ -171,7 +171,6 @@ export class DebugInstaller
                 "System.Collections.Specialized":  "4.0.1",
                 "System.Collections.Immutable": "1.2.0", 
                 "System.Diagnostics.Process" : "4.1.0",
-                "System.Diagnostics.StackTrace":  "4.0.1",  
                 "System.Dynamic.Runtime": "4.0.11",
                 "Microsoft.CSharp": "4.0.1",
                 "System.Threading.Tasks.Dataflow": "4.6.0",