docs: remove sponsor section and sponsor assets from project docs

[Copilot]

The issue requested removing sponsor content from the repository. This change removes sponsor promotion from public documentation and deletes related bundled sponsor image assets.

• Documentation cleanup

  • Removed the ## 赞助商 section from README.md
  • Removed embedded external sponsor link and promotional copy tied to that section

• Repository content cleanup

  • Deleted unused sponsor assets under sponsors/:
    • sponsors/shenfengwl.png
    • sponsors/91api.jpg

- ## 赞助商
- <div align="center">
-   ...
- </div>
- 目前该项目流量较大...我会在项目中加入您的广告...

[Copilot]

Pull request overview

Removes sponsor promotion content from the public-facing documentation, aligning the repository with the request to eliminate sponsor materials.

Changes:

• Removed the ## 赞助商 section (including embedded HTML, external link, and promotional copy) from README.md.
• Removed bundled sponsor image assets by deleting the sponsors/ directory contents (directory no longer present at repo root).

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