Remove clear from NuGet.config

NuGet.config

@@ -1,7 +1,6 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <clear/>
     <add key="NUnit AppVeyor CI" value="https://ci.appveyor.com/nuget/nunit" />
     <add key="NUNit MyGet Feed" value="https://www.myget.org/F/nunit/api/v2" />
     <add key="nuget.org" value="https://www.nuget.org/api/v2/" />