executor: remove Next() function for ShowDDLExec (#6059)
executor: remove Next() function for ShowDDLExec (#6059)


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