Revert "Add dotnet-public to test NuGet.Config (#5195)"

This reverts commit 4893ce2822.

tests/Microsoft.DotNet.Docker.Tests/TestAppArtifacts/NuGet.config

@@ -1,7 +1,2 @@
 <?xml version="1.0" encoding="utf-8"?>
-<configuration>
-  <packageSources>
-    <clear />
-    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
-  </packageSources>
-</configuration>
+<configuration></configuration>