Revert 95d737ff to add 'ignore_nulls'

Per discussion, revert the commit which added 'ignore_nulls' to
row_to_json.  This capability would be better added as an independent
function rather than being bolted on to row_to_json.  Additionally,
the implementation didn't address complex JSON objects, and so was
incomplete anyway.

Pointed out by Tom and discussed with Andrew and Robert.