remove MERGE FIXME in function ExecAddMatchingSubPlans (#13381)
remove MERGE FIXME in function ExecAddMatchingSubPlans (#13381)

In the master branch, Partition Selector is implemented by ExecAddMatchingSubPlans(),
which computes a subset of eligible partitions, this will be used by the outer table filtering
of the hash join.

And reuse pg's function ExecFindMatchingSubPlans, find all subpartitions that match
the pruning steps. Also returns a heap-allocated bitmap that stores the subpartition where
the tuple is located. Therefore, if we want to collect all subsets of partitions, we have to
keep calling ExecAddMatchingSubPlans in Seq Scan to add existing subpartitions to the
final result, and because ExecFindMatchingSubPlans returns heap memory objects, we
have to active free, which does bring some performance overhead.

But on the one hand, we don't have a way to get all the partition subsets (volcano model)
at once, and we can't modify the ExecAddMatchingSubPlans function.

So I think the current method is the only implementation, so remove the fixme.
xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb