Bug 1085290 - [Bluetooth][Settings] Support version detection for Bluetooth module/panel between v1/v2 API

Good news! I just found that `this._bluetooth` is not required. The reason is when we resolve the promise, it keeps the resolved result. So when someone is trying to chain a resolved promise, it calls to the callback immediately, which is exactly what we want here.

In short: we could remove `this._bluetooth` and the related check.