Clean a10networks_ax_directory_traversal

Hi @xistence, this pull request jus tries to clean up https://github.com/rapid7/metasploit-framework/pull/2926, just small fixes. Please feel free to review, check and land once you feel comfortable, https://github.com/rapid7/metasploit-framework/pull/2926 will be automatically updated. In the meanwhile I'd like to ask if any additional measures should be taken, because of the target file being deleted, even when there is a warning on the description. Thanks a lot for your collaboration!