[PE] remove stale commit signing instructions

[pq]

Since we're not requiring signing anymore, we can remove this bit from the contributing docs.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.

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