kv/union_store:remove some unused type (#23620)
kv/union_store:remove some unused type (#23620)

 
ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,pingcap/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,coocood/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,coocood/tidb,c4pt0r/tidb,pingcap/tidb