[test](regression) Remove no simdjson flexible update case

[bobhan1]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: The regression suite still had test_f_no_simdjson, which duplicated the flexible partial update JSON coverage only to force enable_simdjson_reader=false. Since enable_simdjson_reader is obsolete, this separate docker regression case no longer provides useful coverage and only adds regression cost.

This PR removes the obsolete test_f_no_simdjson case.

Release note

None

Check List (For Author)

• Test: No need to test (removes an obsolete regression case only)
  • Manual test: rg -n "test_f_no_simdjson|enable_simdjson_reader" regression-test/suites/unique_with_mow_p0/flexible confirmed no remaining references
  • Manual test: git diff --check upstream/master..HEAD
• Behavior changed: No
• Does this need documentation: No

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

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?

[bobhan1]

run buildall

[bobhan1]

run external

[bobhan1]

/review

[github-actions]

I found one cleanup issue before this can be approved: the suite deletion leaves its generated regression output file in the tree.

Critical checkpoint conclusions:

• Goal/test: the PR removes the obsolete no-simdjson flexible partial-update docker suite. The remaining test_flexible_partial_update suite covers the same JSON stream-load scenarios on the current reader path.
• Scope: the change is small and focused, but the paired generated result artifact was not removed.
• Concurrency/lifecycle/config/compatibility/FE-BE/persistence/data writes: no runtime code changes, so these are not applicable.
• Parallel paths: no missing runtime parallel-path update found; this is a test-only deletion.
• Test coverage/results: removing this obsolete duplicate is reasonable, but regression-test/data/unique_with_mow_p0/flexible/test_f_no_simdjson.out is now orphaned and should be deleted with the suite.
• Observability/performance/security: not applicable for this test-only removal.
• User focus: no additional user-provided focus points were supplied.

Review checks performed: inspected AGENTS.md, the repository code-review skill, existing review context, the full PR diff, related remaining flexible partial-update coverage, tracked output artifacts, and git diff --check.

[github-actions]

Deleting this suite should also delete its generated result file. regression-test/data/unique_with_mow_p0/flexible/test_f_no_simdjson.out still exists in the PR head, but no test_f_no_simdjson suite remains. Doris regression outputs are generated artifacts paired with suites; leaving this orphaned .out makes the removed test look like it still has expected results and creates stale data for future output updates. Please remove that .out file together with this suite.