MB-12476: Spatial views don't support real reduce

Spatial views don't support real reduce. The reduce value is used
to store the "original MBB" which is needed by the RR*-tree algorithm.
It is the MBB the node had on creation time.

Change-Id: I42102f22af7207c883cdec5a8f10e923f318f49b
Reviewed-on: http://review.couchbase.org/42758
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>