EZP-21825: Temporary files don't always get cleaned up, filling up disk space

Temporary files could be left on the local FS when the copy from DFS to FS is considered unsuccessful.

In `eZDFSFileHandlerMySQLiBackend::_fetch()` the filename of the temporary file changes in every iteration of the loop, but gets deleted only after the loop.

So, when the loop gets iterated 3 times and the copy process is still unsuccessful, 2 temporary files remain on the file system.

https://jira.ez.no/browse/EZP-21825

Cheers
:octocat: Jérôme
Oh my, it has been so long, I try to remember :). I think I added this there as a final precaution to not have obsolete temp files on the file system. But logically, this shouldn't be needed at this point anymore. Should I remove it? I don't have access to the project anymore on which this issue occurred, so unfortunately, I can't test it anymore.