store/copr: revert pr/35975, do not reduce concurrency for keep order coprocessor request (#55141)

[ti-chi-bot]

This is an automated cherry-pick of #55141

What problem does this PR solve?

Issue Number: close #54969

Problem Summary:

What changed and how does it work?

In the past, #35975 reduce coprocessor concurrency to 2 for the keep order query.
But recently we meet an oncall case that the change cause performance regression.

Balance the memory usage and concurrency is a difficult job, we need more tests and expirements to reach the conclusion.
Currently we can simply set TiFlash to not reduce concurrency, when a user using TiFlash, the tidb memory usage would not be a top concern (their hardware must be better).

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

See comments below

• No need to test

  • I checked and no code files have been changed.

One line change, just revert part of the previous optimization.

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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-6.5@d36d14a). Learn more about missing BASE report.

Additional details and impacted files

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[tiancaiamao]

This is not a correct fix, use #55645 instead