[CI Bugfix] Fix CI by fully removing --enable-prompt-adapter

[mgoin]

Purpose

Fix CI failure introduced by #23190

Test Plan

Test Result

(Optional) Documentation Update

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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.

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[gemini-code-assist]

Code Review

This pull request addresses a CI failure by removing the deprecated --enable-prompt-adapter command-line argument. The change is confined to vllm/engine/arg_utils.py and is a clean removal of a no-op argument. I have verified that there are no remaining usages of this argument within the provided codebase, making this a safe and correct change. I approve of this fix.