[iOS] Only use __enable_native_promise__ option when explicitly set i…

[wqyfavor]

…n config.

First of all, thank you for your contribution!

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

CheckList:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update test scripts for the feature.
• Add unit tests for the feature.

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