fix error while downloading dependencies for CPU backend

[MaoJianwei]

Purpose

remove the --extra-index-url link, to fix the following error while downloading dependencies before building VLLM for CPU backend.

Test Result

Download dependencies ok.
Build VLLM ok.

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• 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
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[gemini-code-assist]

Code Review

This pull request resolves a dependency installation error for the CPU backend by removing the --extra-index-url option from requirements/cpu-build.txt and requirements/cpu.txt. While pip can support this option in requirements files, it was causing errors in the author's environment. Removing it makes the installation process more robust by relying on command-line flags or environment variables to specify the PyTorch package index, which is already the documented practice for this project. The change is correct and improves the consistency of the installation process.