Remove '--cpu' option from ExecStart command

[sethgregory]

By default, the ExecStart in the system service includes the --cpu flag which forces ComfyUI to run entirely on CPU even if a GPU is present. This commit removes that flag.

✍️ Description

Unless there's a compelling reason for --cpu to be included in the ExecStart by default, this should be removed as it prevents the GPU from being utilized even if it is present.

🔗 Related Issue

Fixes #

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.