Revert "MB-11472 Avoid btree update call when there is nothing to flush"

This reverts commit d7d99c5d2b42eac9b4268899076dd0b3f2e0d464.
It is identified that this commit causes data corruption (MB-11486)

Change-Id: I3337cd5f368853710c17bcd11ef1d85ad59daceb
Reviewed-on: http://review.couchbase.org/38647
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>