fix(qwen-image): drop the shadowed dead _get_qwen_prompt_embeds duplicate

[Anai-Guo]

Problem

QwenImageEditPipeline._get_qwen_prompt_embeds is defined twice in
vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit.py. In Python
the second def silently shadows the first, so the first 44-line copy is dead
code that can never run.

The two definitions are not interchangeable:

• The first (dead) copy has signature (self, prompt, image, dtype) and no
  sequence-length validation.

• The second (live) copy adds max_sequence_length / prompt_name, calls
  validate_prompt_sequence_lengths(...), and is what encode_prompt actually
  invokes:

  prompt_embeds, prompt_embeds_mask = self._get_qwen_prompt_embeds(
      prompt, image,
      max_sequence_length=max_sequence_length,
      prompt_name=prompt_name,
  )

  Those keyword arguments only exist on the second definition — the first would
  raise TypeError if it were ever bound.

Fix

Remove the unreachable first definition. This is a pure no-op at runtime (the
live method is untouched) and drops a latent footgun.

+0 / -45, no behavior change.

🤖 Generated with Claude Code

[vllm-omni-review-bot]

This PR appears to belong to: docs/design/module/diffusion/offloader.md, docs/design/module/diffusion/diffusion_model_integration.md, docs/design/module/diffusion/index.md.

Module owners: @wtomin @david6666666 @Isotr0py

Routing: @wtomin via module of the changed files, module named in the PR description, CODEOWNERS; @david6666666 via module of the changed files, module named in the PR description; @Isotr0py via module of the changed files, module named in the PR description

@Anai-Guo, please review your own changes and leave a short self-review comment describing what you checked. PRs without author self-review may not be assigned a reviewer.

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[NickCao]

LGTM, the shadowed first copy is never reachable.