refactor(fromFetch): some clean up on the implementation (#5782)
refactor(fromFetch): some clean up on the implementation (#5782)

* refactor(fromFetch): some clean up on the implementation

* chore: update comment from feedback

https://github.com/ReactiveX/rxjs/pull/5782\#discussion_r497952764
ReactiveX/rxjs,benlesh/RxJS,martinsik/rxjs,ReactiveX/RxJS,ReactiveX/rxjs,martinsik/rxjs,martinsik/rxjs,benlesh/RxJS,benlesh/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,ReactiveX/RxJS,benlesh/RxJS,martinsik/rxjs