pgsql: Fix inheritance_planner() to delete dummy subplans from its

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/optimizer/plan:
        planner.c (r1.206 -> r1.207)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c.diff?r1=1.206&r2=1.207)