syncrepo tweaks now that rsync has fixed the stale .~tmp~ issue

Some quick changes since we can rely on rsync a bit more again:

• Remove the .tmp workaround from our own mirrors (I have updated rsync on all of them)
• Tweak rsync options a bit (no functional change other than replacing --delete-after with --delete-delay; the latter computes deletions during the transfer and should avoid doing a separate pass)
• Add a note to the syncrepo template that the .tmp workaround can be ignored if using a (yet unreleased) rsync version with a fix for this issue

And while we're at it:

• Don't make use of a temporary directory for file transfers (reasoning explained in the commit)