perf(nuxt): tree-shake asyncData client logic from server

[pi0]

🔗 Linked issue

Extracted from #5738 by @OhB00

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Move the client-only logic to process.client scope so that we are sure bundler can tree-shake it for server also increase code readability since is only used in if condition block.

Co-authored-by: Ohb00 43827372+OhB00@users.noreply.github.com

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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