Simplify flatten

Simplify `IterableOps#flatten` so that implementations only need to override `flatMap`

Author: NthPortal

library/src/scala/collection/Iterable.scala

@@ -672,8 +672,7 @@ trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] with Iterable
 
  def flatMap[B](f: A => IterableOnce[B]): CC[B] = iterableFactory.from(new View.FlatMap(this, f))
 
- def flatten[B](implicit asIterable: A => IterableOnce[B]): CC[B] =
- iterableFactory.from(new View.FlatMap(this, asIterable))
+ def flatten[B](implicit asIterable: A => IterableOnce[B]): CC[B] = flatMap(asIterable)
 
  def collect[B](pf: PartialFunction[A, B]): CC[B] =
  iterableFactory.from(new View.Collect(this, pf))