Skip deletion of files in clearFiles().

This fixes an issue where Spark could delete
original files in the current working directory
that were added to the job using addFile().

There was also the potential for addFile() to
overwrite local files, which is addressed by
changing Utils.fetchFile() to log a warning
instead of overwriting a file with new contents.

This is a short-term fix; a better long-term
solution would be to remove the dependence on
storing files in the current working directory,
since we can't change the cwd from Java.