Remove overly-pessimistic statement about constraint exclusion.
Remove overly-pessimistic statement about constraint exclusion.

As of 9.2, constraint exclusion should work okay with prepared statements:
the planner will try custom plans with actual values of the parameters,
and observe that they are a lot cheaper than the generic plan, and thus
never fall back to using the generic plan.  Noted by Tatsuhito Kasahara.

Postgres-XL/Postgres-XL,xinzweb/gpdb,Postgres-XL/Postgres-XL,greenplum-db/gpdb,lisakowen/gpdb,ovr/postgres-xl,pavanvd/postgres-xl,zeroae/postgres-xl,oberstet/postgres-xl,lisakowen/gpdb,ovr/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,jmcatamney/gpdb,50wu/gpdb,techdragon/Postgres-XL,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,yazun/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,50wu/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,techdragon/Postgres-XL,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,50wu/gpdb,xinzweb/gpdb,yazun/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,oberstet/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,50wu/gpdb,ovr/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,Postgres-XL/Postgres-XL,pavanvd/postgres-xl,techdragon/Postgres-XL,xinzweb/gpdb,lisakowen/gpdb,ovr/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,50wu/gpdb,yazun/postgres-xl,zeroae/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb