Remove support for `this = draft` in recipes

[mweststrate]

Currently, this is always equal to the draft inside recipes. I suggest to remove that. It has a few benefits:

1. simpler typings
2. simpler docs
3. I didn't see anyone use it
4. it makes pattern like this possible (note that we should make sure that the curried version does preserve this):

const context = {
  foo: 1,
  bar: produce(function(draft) {
    draft.foo = this.foo
  })
}

[scriby]

+1 to this

[mweststrate]

Done in the immer3 branch.