Remove max_txn_size engine parameter

We no longer use this variable and as a result we should remove it.

Change-Id: I75d49cd351590cdeffbf2c1908fff4a70cf388cc
Reviewed-on: http://review.couchbase.org/34375
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>