Remove references to deprecated MONGO_REPLICA_SETS

Remove references in the default broker.conf and documentation to the
MONGO_REPLICA_SETS setting, which was removed in commit
2449f2e62b47f8a7e1e69ea0c1db7419b82a167e and commit
4346b2c98717cd7f264973e1c2e2e206abef471c.  These references were
erroneously added in commit e4f862733adf9a5ed75dc3065a584a83de1797b8.