fix(top-slowquery): fix db filter sometimes can not select item

[baurine]

This PR fixes the bug that db filter can not select item after changing the time window, and clean up some codes.

Screenity video - May 23, 2024.webm

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shhdgit

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/pingcap/tidb-dashboard/blob/master/OWNERS)~~ [shhdgit] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-05-23 09:23:18.210591205 +0000 UTC m=+2336351.967726777: :ballot_box_with_check: agreed by shhdgit.