fix(css): sourcemap crash with postcss (fix #7978)

[sapphi-red]

Description

This crash occured because removing <no source> was illegal.

I wrote this line based on a hypothesis that source maps will be combined incorrectly if multiple <no source> exists.
https://github.com/vitejs/vite/pull/7173/files#diff-2cfbd4f4d8c32727cd8e1a561cffbde0b384a3ce0789340440e144f9d64c10f6R834-R835
But since that happens rarely and a crash should be avoided, I think it I should remove this line.

fixes #7978

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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