Remove audit sources from NuGet.config (#12641)

The api.nuget.org source will clash with the CFSClean network isolation
policy and the same data is provided by AzDO now.

Author: akoeplinger

NuGet.config

@@ -60,8 +60,4 @@
     <!--  End: Package sources from dotnet-runtime -->
     <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
   </disabledPackageSources>
-  <auditSources>
-    <clear />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-  </auditSources>
 </configuration>