HIVE-8053: Disable hive.optimize.union.remove when hive.execution.engine=spark [Spark Branch] (Na via Xuefu)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1625203 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'upstream/spark' into spark