fix - remove missing file DESIGN.md link

[ruseel]

DESIGN.md does not exists in main, So I removed it.

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[gemini-code-assist]

Code Review

This pull request removes a reference and link to DESIGN.md from the packages/genui/README.md file. There are no review comments, and I have no additional feedback to provide.

_{Note: Security Review has been skipped due to the limited scope of the PR.}