fix: warn about dynamic import polyfill only during build

[patak-dev]

Description

Since we are enabling the dynamic import polyfill only during build, we can not reliably warn about an unneeded import 'vite/dynamic-import-polyfill'. We can also not detect the legacy plugin, since the user may want to enable this plugin only without the full legacy build. This PR removes this warning. See related discussion in #3434

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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[patak-dev]

We can actually safely warn during build, I restored the warning with this check