drop self-named column aliases in e2e datetime range query

Aliasing toString(col) back to the column's own name makes older ClickHouse
(LTS) resolve the expression against the alias and return empty; the test
only reads columns positionally, so the aliases are unnecessary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dtunikov

flow/e2e/clickhouse_mysql_test.go

@@ -1809,7 +1809,7 @@ func (s ClickHouseSuite) Test_MySQL_DateTime_ClickHouse_Range() {
 		"d_null_low", "d_null_high", "d_nn_low", "d_nn_high", "d_ok",
 		"dt_null_low", "dt_null_high", "dt_nn_low", "dt_nn_high", "dt_ok",
 	} {
-		colExprs = append(colExprs, fmt.Sprintf("ifNull(toString(%s),'') AS %s", c, c))
+		colExprs = append(colExprs, fmt.Sprintf("ifNull(toString(%s),'')", c))
 	}
 	cols := strings.Join(colExprs, ",")
 	rows, err := s.GetRows(dstTableName, cols)