Fix hash aggregation to suppress unneeded columns from being stored in

Fix hash aggregation to suppress unneeded columns from being stored in
tuple hash table entries.  This addresses the problem previously noted
that use of a 'physical tlist' in the input scan node could bloat the
hash table entries far beyond what the planner expects.  It's a better
answer than my previous thought of undoing the physical tlist optimization,
because we can also remove columns that are needed to compute the aggregate
functions but aren't part of the grouping column set.