Using a temporary directory outside of /srv/ftp was meant to protect
against incomplete files from being synced by downstream mirrors. It
does not achieve this to much effect though; each file gets uploaded
to the temporary directory but then immediately moved under a .~tmp~
directory at its target location (.~tmp~ because of --delay-updates,
otherwise the file would be renamed to its final path).

The `--delay-updates` option by itself sufficiently protects against
temp files being transferred to downstream mirrors; when used by the
receiver, it automatically adds an exclude rule for ~.tmp~, behaving
exactly like we want it to. As such, the `--temp-dir` option doesn't
provide any further benefit and can be removed.