[Misc] Remove the duplicate code

[chaunceyjiang]

Purpose

When implementing #24317, #26706 had not yet been merged. Now that #26706 is merged, this piece of code is no longer needed.

Test Plan

A complete end-to-end test was written in #24317, which can verify that the functionality remains correct after removing this code.

Test Result

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly removes a redundant block of code from _construct_input_messages_with_harmony. The removed code, which converted a dict to a ResponseFunctionToolCall object, is now handled by the function_call_parsing model validator in the ResponsesRequest class. This change is a good cleanup that improves maintainability by eliminating duplicated logic. The change is correct and I have no further suggestions.