Remove reference to myget.org as a package resource (#1640)

This is disallowed by policy

Author: JamesWTruher

NuGet.Config

@@ -3,6 +3,5 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="powershell-core" value="https://powershell.myget.org/F/powershell-core/api/v3/index.json" />
   </packageSources>
 </configuration>