Add clear statement (dotnet#6557)

* Add clear statement

* Remove additional package source

Author: mairaw

samples/nuget/nuget.config

@@ -1,6 +1,6 @@
 <configuration>
   <packageSources>
+    <clear/>
     <add key="dotnet6" value="https://dnceng.pkgs.visualstudio.com/public/_packaging/dotnet6/nuget/v3/index.json" />
-    <add key="gRPC repository" value="https://grpc.jfrog.io/grpc/api/nuget/v3/grpc-nuget-dev" />
   </packageSources>
 </configuration>