🧹 Remove unused imports from VolSurfaceAPI

[sharesth23]

🎯 What: Removed unused imports butterfly_arbitrage and calendar_arbitrage from volsurface/api.py.
💡 Why: To improve code health by cleaning up the namespace and removing unnecessary dependencies from the file, leading to better readability and maintainability.
✅ Verification: Verified that the removal of these imports does not affect functionality by successfully running the full test suite (pytest volsurface/tests/), where all 18 tests passed. Code review also confirmed the change is safe and effective.
✨ Result: A cleaner file with no unused imports, making the codebase easier to maintain without altering any behavior.

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PR created automatically by Jules for task 9391503567187217719 started by @sharesth23

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[sharesth23]

Unused imports being deleted from the code