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

Log Message:
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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:
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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)