Do not unlink file while it is still open

- inside RunJob() there is a deferred function which will remove
  the entire temporary directory in which images are created, including
  the streamOptimized file

- inside testBootUsingVMware(), which wants to use this function,
  there is already a deferred function which removes the converted
  image

internal/upload/vmware/vmware.go

@@ -18,9 +18,5 @@ func OpenAsStreamOptimizedVmdk(imagePath string) (*os.File, error) {
 	if err != nil {
 		return nil, err
 	}
-	err = os.Remove(newPath)
-	if err != nil {
-		return nil, err
-	}
 	return f, err
 }