MB-7828 Don't attempt to copy purged docs list

We don't have a purged docs list (but we have started using the old
purge_seq field), and some old code has broken. Removing it.

Change-Id: I1b376920db2acddac595acbab0adb762fd22de44
Reviewed-on: http://review.couchbase.org/24885
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Reviewed-by: Damien Katz <damien@couchbase.com>