Remove FB.napi

[dantman]

For some time this library has supported an alternative FB.napi which uses a Node style callback instead of the one Facebook uses.

I generally do not know if anyone actually benefits from this anymore:

• If you are trying to write something cross-compatible with the Facebook client-side SDK, you use the FB.api(..., callback) format since that is the only thing that works in both.
• Promises are quickly replacing node-style callbacks, I doubt many people are using old libraries like step when they can instead just use Promises and even take advantage of async/await which is built into the language now.

However this is a big breaking change that will force people to refactor their code if it turns out there is a segment of users that still uses the node style callbacks.

Thus my current plan is to release 3.0.0 with modification to napi that emits a warning when used saying that we plan to remove this api and pointing to this bug report if people want it.

If I do not get a significant number of people commenting in favour of keeping napi in this bug, I will remove napi in 4.0.0.