MB-8877 Fix builtin reduces for subset development views

The regression was introduced in the commit
912b8667560a275a652a2ff0d9d9264ed38e9328:

"MB-7978 Fixes to view tree with multiple reducers"

It only affects 2.1.0, 2.1.1 and 2.2.0 releases. The 3.0 (master branch)
is unaffected, because the same view engine is used for both development
and production views, unlike previous releases.

Change-Id: Id2161d98b015a4fb8a841ded38affad31e4a8c56
Reviewed-on: http://review.couchbase.org/28429
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>