[test](mv) Remove 'set enable_stats=false' in sync mv regression test to make mv regression test case stable

[seawinde]

What problem does this PR solve?

In the regression testing for synchronous materialized views, many cases specify set enable_stats=false. This can result in row counts being reported as unknown, causing the Cost-Based Optimizer (CBO) to inconsistently choose query plans that include materialized views. In real‑world environments, enable_statsdefaults to true. Therefore, to stabilize the pipeline and fix the flakiness of synchronous materialized view test cases, we are removing set enable_stats=false from the tests.

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• 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 code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

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

[hello-stephen]

Thank you for your contribution to Apache Doris.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[seawinde]

run buildall

[hello-stephen]

LGTM

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.