Fix parse_agg.c to detect ungrouped Vars in sub-SELECTs; remove code

Fix parse_agg.c to detect ungrouped Vars in sub-SELECTs; remove code
that used to do it in planner.  That was an ancient kluge that was
never satisfactory; errors should be detected at parse time when possible.
But at the time we didn't have the support mechanism (expression_tree_walker
et al) to make it convenient to do in the parser.