Remove sorting step for mapreduce initial index builder

Native btree index builder takes care of sorting before
the btree build. Hence sorting step is not required from
erlang side for mapreduce views.

Change-Id: Iafee54d0d65ca4c5cd90389d91e258792776989e
Reviewed-on: http://review.couchbase.org/32550
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>