statistics: fix async merge global stats use cmsketch twice (#59198)

[ti-chi-bot]

This is an automated cherry-pick of #59198

What problem does this PR solve?

Issue Number: close #59194

Problem Summary:
the same cmsketch is merge twice, which affects row estimation.

What changed and how does it work?

remove the unnecessary part.

Check List

Tests

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

  • I checked and no code files have been changed.

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

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-8.5@e94d8dc). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #59209   +/-   ##
================================================
  Coverage               ?   56.8992%           
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  Files                  ?       1769           
  Lines                  ?     627455           
  Branches               ?          0           
================================================
  Hits                   ?     357017           
  Misses                 ?     246305           
  Partials               ?      24133           

Flag	Coverage Δ
integration	36.7810% <ø> (?)
unit	72.5895% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.8711% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	52.2807% <0.0000%> (?)