Drop with after class

[odersky]

Disallow with after class. It was still allowed before by accident since
the relevant code slipped through a revert commit.

[bishabosha]

Approve - all the following were rejected, as expected:

class Foo with
  def foo = 23

trait Bar with
  def bar = 23

enum Baz with
  case A, B, C

object Qux with
  def qux = 23

def nested = {
  object Nested with
    def quxx = 47
}