[optimization] removed unused List and related variables

[dk2k]

What problem does this PR solve?

Issue Number: close #issue_number

Problem Description: TBD

What is changed and how does it work?

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has methods of interface change
• Has persistent data change
• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

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

Side effects

• Possible performance regression, WHY: TBD
• Increased code complexity, WHY: TBD
• Breaking backward compatibility, WHY: TBD
• NO side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note
• NO related changes

[ti-chi-bot]

Welcome @dk2k! It looks like this is your first PR to tikv/client-java 🎉

[ti-chi-bot]

Thanks for your pull request. Before we can look at it, you'll need to add a 'DCO signoff' to your commits.

📝 Please follow instructions in the contributing guide to update your commits with the DCO

Full details of the Developer Certificate of Origin can be found at developercertificate.org.

The list of commits missing DCO signoff:

• ae7126e Removed unused List
• 3fa6eab Removed unused variable
• 19b487a Removed unused variable

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