fix mutil_uniq (#2650)

[ti-chi-bot]

This is an automated cherry-pick of #2650

What problem does this PR solve?

close #2649

What is changed and how it works?

The behavior of other version/client

1. TiSpark 3.0 does not pass unique in indexscan executor
2. TIDB needs some checks before passing unique https://github.com/pingcap/tidb/blob/4ae5be190b0017675deedd9af8b80d8868e03f0e/planner/core/plan_to_pb.go#L271

TiSpark lacks the necessary information to judge if it can pass unique. So we just delete it. Lack unique in indexscan only slows the performance but does not influence the correct. See the pr's comment for more details https://github.com/pingcap/tidb/issues/29650

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Daemonxiao
• shiyuhang0

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[shiyuhang0]

/run-all-tests

[shiyuhang0]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5a59e666b75a8dd49a755da88ca803afd1251c71