statistics: remove useless code in the MergePartTopN2GlobalTopN (#45755)

[ti-chi-bot]

This is an automated cherry-pick of #45755

What problem does this PR solve?

Issue Number: close #45706

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@df3fa99). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-6.5     #45770   +/-   ##
================================================
  Coverage               ?   74.3830%           
================================================
  Files                  ?       1083           
  Lines                  ?     351759           
  Branches               ?          0           
================================================
  Hits                   ?     261649           
  Misses                 ?      73805           
  Partials               ?      16305           

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qw4990, time-and-fate

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [qw4990,time-and-fate]

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]

[LGTM Timeline notifier]

Timeline:

• 2023-08-10 01:51:53.616689889 +0000 UTC m=+163878.165705861: ☑️ agreed by time-and-fate.
• 2023-08-10 01:53:07.773485904 +0000 UTC m=+163952.322501890: ☑️ agreed by qw4990.