Remove Validated.filter

I think this was leftover from copy/paste from `Xor`. It mentions that
it's useful when using `Validated` in for-comprehensions, but you can't
actually use `Validated` in for-comprehensions since it doesn't have a
`flatMap` method :).

Also, `Xor` no longer has a `filter` method. It was removed in typelevel#276
since using the monoid zero when the predicate doesn't match is rather
arbitrary and can lead to surprises. I think we should follow that
precedent for `Validated`.

core/src/main/scala/cats/data/Validated.scala

@@ -38,15 +38,6 @@ sealed abstract class Validated[+E, +A] extends Product with Serializable {
    */
   def forall(f: A => Boolean): Boolean = fold(_ => true, f)
 
-  /**
-   * If the value is Valid but the predicate fails, return an empty
-   * Invalid value, otherwise leaves the value unchanged.  This method
-   * is mostly useful for allowing validated values to be used in a
-   * for comprehension with pattern matching.
-   */
-  def filter[EE >: E](pred: A => Boolean)(implicit M: Monoid[EE]): Validated[EE,A] =
-    fold(Invalid.apply, a => if(pred(a)) this else Invalid(M.empty))
-
   /**
    * Return this if it is Valid, or else fall back to the given default.
    */