HIVE-4927 : When we merge two MapJoin MapRedTasks, the TableScanOperator of the second one should be removed (Yin Huai via Ashutosh Chauhan)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1507359 13f79535-47bb-0310-9956-ffa450edef68