Simplify max seq numbers calculation

Merging 2 lists of sequence numbers (by maximum) is
already performed by the function process_map_results/3.

Change-Id: I1728dc55ce2a34d3c148bbaa7f6f73e89a69d328
Reviewed-on: http://review.couchbase.org/25012
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>