TiDB gets row data even using covered index

## Enhancement
create table t1(id1 int, id2 int, id3 int, PRIMARY KEY(id1), UNIQUE KEY udx_id2 (id2));

SELECT id2 FROM t1 WHERE id2 = 1

The point-get statement above used covered index `udx_id2 `, but TiDB still needs to get the row data. In fact, we should remove this step.