Clean up Outdated Args from Llama Export Script

[YifanShenSZ]

2 coreml-specific args were introduced to llama export script

1. Partition Mutable Buffer as Core ML State #5165 introduced --coreml-enable-state
2. Preserve SDPA for CoreML #5258 introduced --coreml-preserve-sdpa

Since users would want to target iOS version rather than tune optimizations, #5319 introduced --coreml-ios 18. However, #5319 forgot to remove the outdated --coreml-enable-state and --coreml-preserve-sdpa

[pytorch-bot]

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[YifanShenSZ]

No need to cherry pick this PR, since it won't affect any beta release functionality