Remove deprecated command from build_runner.md

[vbuberen]

This PR removes deprecated flutter run command from build_runner description page.

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• This PR doesn’t contain automatically generated corrections or text (Grammarly, LLMs, and similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
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[parlough]

Thank you very much!