Revert "MB-10348: Vbucket's purge_seqno available in stats"

This reverts commit 70670dc64eaabb3300235569a1e0edcb936168e7.

Change-Id: Idf4467f083793e3eaf1e6b097f8822502c13f4d3
Reviewed-on: http://review.couchbase.org/35174
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>