MB-11945 Remove the assertion check on sequence number in the flusher.

We sporadically observe the sequence number mismatches between the
flusher and Couchstore. This is not a regression from any recent
changes, and mostly existed much earlier. We will investigate this
issue separately to see if there are any bugs in ep-engine or
Couchstore.

Change-Id: Ifac2eae0deae09cad09dd7b5664f63bcbf828b24
Reviewed-on: http://review.couchbase.org/40630
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>