HIVE-4877 : In ExecReducer, remove tag from the row which will be passed to the first Operator at the Reduce-side (Yin Huai via Ashutosh Chauhan)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1505233 13f79535-47bb-0310-9956-ffa450edef68