Cleanup NuGet.Config's to ensure we only reference released packages and our build-related ones

Ix.NET/Source/NuGet.Config

@@ -1,13 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <clear />
-    <add key="myget.org dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
     <add key="Build Packages" value="https://www.myget.org/F/c037199d-41df-4567-b966-25ff65324688/api/v3/index.json" />
-    <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />    
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
+    <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
-  <config>
-    <!--<add key="repositoryPath" value="..\..\packages" />-->
-  </config>
 </configuration>

Rx.NET/Source/NuGet.Config

@@ -1,12 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <clear />
     <add key="Build Packages" value="https://www.myget.org/F/c037199d-41df-4567-b966-25ff65324688/api/v3/index.json" />
-    <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />    
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
+    <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
-  <disabledPackageSources>
-     <clear />
-  </disabledPackageSources>
 </configuration>