SourceBackend: don't delete old files in bundle directory

There might be some files the compiler don't know about (copied via UXL), and
this is causing the compiler to delete some files and output the same files
again and again, every time we build...

The user can clean before building to be sure to remove old files.

src/compiler/Uno.Compiler.API/Backends/SourceBackend.cs

@@ -76,8 +76,6 @@ protected virtual void ExportBundle(string directory)
 
             foreach (var file in Data.Extensions.BundleFiles)
                 Disk.CopyFile(file.SourcePath, Path.Combine(directory, file.TargetName));
-
-            Disk.DeleteOutdatedFiles(directory);
         }
 
         protected virtual void ExportNamespace(Namespace ns)