`sys.schema_unused_indexes` doesn't work well

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE t (id int primary key, col int unique);
SELECT * FROM sys.schema_unused_indexes;

2. What did you expect to see? (Required)

1. The rows should include col;
2. The rows shouldn't include primary;

The first is because of some execution issue in tidb coprocessor (but I'm not sure, the plan looks good).

The second is because the primary should be PRIMARY. I should modify it in the view definition.

3. What did you see instead (Required)

It only has one row: primary.

4. What is your TiDB version? (Required)