Perhaps `downloadFile()` does not need `begin` hook

[birdofpreyru]

The hook seems to return almost the same info as the downloadFile() promise, and also it is fired at about the same time the promise is resolved. Thus, perhaps, drop the hook, add to the promise any info which is in the hook, but not in the promise.

For additional context see https://github.com/birdofpreyru/react-native-fs/pull/18#issuecomment-1802896373