executor: remove Next function for UnionScanExec (#5996)
executor: remove Next function for UnionScanExec (#5996)



CodeRushing/tidb,qiuyesuifeng/tidb,ngaut/tidb,c4pt0r/tidb,coocood/tidb,XuHuaiyu/tidb,cosmtrek/tidb,mccxj/tidb,Cofyc/tidb,cosmtrek/tidb,ming-relax/tidb,CodeRushing/tidb,kyle-wang/tidb,ngaut/tidb,Nify/tidb,wusphinx/tidb,ngaut/tidb,ngaut/tidb,c4pt0r/tidb,kyle-wang/tidb,wusphinx/tidb,qiuyesuifeng/tidb,ngaut/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,coocood/tidb,Nify/tidb,XuHuaiyu/tidb,pingcap/tidb,pingcap/tidb,EvilMcJerkface/tidb,c4pt0r/tidb,tiancaiamao/tidb,kyle-wang/tidb,pingcap/tidb,tiancaiamao/tidb,mccxj/tidb,c4pt0r/tidb,pingcap/tidb,c4pt0r/tidb,EvilMcJerkface/tidb,ming-relax/tidb,ngaut/tidb,kyle-wang/tidb,Cofyc/tidb,kyle-wang/tidb,pingcap/tidb