Remove reference from test project.

Author: Hosch250

VSDiagnostics/VSDiagnostics/VSDiagnostics.Test/packages.config

@@ -12,5 +12,4 @@
   <package id="RoslynTester" version="1.6.2" targetFramework="net45" />
   <package id="System.Collections.Immutable" version="1.1.36" targetFramework="net45" />
   <package id="System.Reflection.Metadata" version="1.0.21" targetFramework="net45" />
-  <package id="VSDiagnostics" version="1.9.3" targetFramework="net45" />
 </packages>