[Fix](flexible partial update) Don't set default value for TOlapTableIndexSchema.unique_key_update_mode

[bobhan1]

What problem does this PR solve?

Problem Summary:

If we explicitly set default value for field unique_key_update_mode in .thirft file, __isset.unique_key_update_mode will always be true. So OlapTableSchemaParam::init_unique_key_update_mode will fail to set _unique_key_update_mode correctly if it receive a request from old version FE.

Check List (For Committer)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No colde files have been changed.
    • Other reason : has been tested in rolling upgrade env

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

• Release note

  None

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

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[dataroaring]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[bobhan1]

run buildall

[zhannngchen]

If there is no test, plz explain why in your PR description.

[bobhan1]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"