Remove additional autogenerate callback

Author: josevalim

lib/ecto/type.ex

@@ -320,9 +320,8 @@ defmodule Ecto.Type do
   to `field` with the `:autogenerate` flag.
   """
   @callback autogenerate() :: term()
-  @callback autogenerate(term) :: term()
 
-  @optional_callbacks autogenerate: 0, autogenerate: 1
+  @optional_callbacks autogenerate: 0
 
   ## Functions