Don't clear the global nuget config (microsoft#2246)

Author: Raj Seshasankaran

nuget.config

@@ -8,7 +8,6 @@
     <!-- When <clear /> is present, previously defined sources are ignored -->
     <!-- Remove this tag or un-comment the nuget.org source below to restore packages from nuget.org -->
     <!-- For more info, see https://docs.nuget.org/consume/nuget-config-file -->
-    <clear />
     <!-- Intentionally v2, as v3 doesn't yet support list which is used by Update-Environment.ps1 -->
     <add key="vss-package-management" value="https://www.myget.org/F/vss-package-management/api/v2" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />