Fix inheritance_planner() to delete dummy subplans from its Append plan

Fix inheritance_planner() to delete dummy subplans from its Append plan
list, when some of the child rels have been excluded by constraint
exclusion.  This doesn't save a huge amount of time but it'll save some,
and it makes the EXPLAIN output look saner.  We already did the
equivalent thing in set_append_rel_pathlist(), but not here.