Treat 'native-threading' option correctly.

Follow-up: #2609 & [this comment](https://github.com/mozilla/servo/commit/850bd2891de589b95e32dc8f0b59d4043ed1e0a3#commitcomment-6779974)

In this place, I agree that we need not accept the long-hand form of `native-threading` option.
If there is special meaning about we accept the long-hand form of it, I'll revert https://github.com/saneyuki/servo/commit/ad5457ba4b75e98360df05cca4d7d438aa3558c2.