MDL-57002 repository_webdav: Use default temporary file name
MDL-57002 repository_webdav: Use default temporary file name

The WebDAV library does not like when there are UTF-8 characters
in the temporary file name. It changes the path but does not return
it, so the file ends up being saved somewhere unknown. As we do not
need the file name at all, we let repository::prepare_file()
generate one.

dilawar/moodle