Do not send delete = TRUE to the drush rsync call even if no-delete

isset. Send it only if no-delete is not set.
This still leaves delete = TRUE as the default, unless no-delete has been set
to TRUE. This solves critical bug #976300.
I can't imagine a case where someone would want to set no-delete = FALSE if
--delete is the default at all other times, but I added a condition to account
for this too.