Late-breaking API correction

[Brian Goetz]

In a total flub, we managed to screw up the signature for one of the 
variants of Comparator.thenComparing:

   <U extends Comparable<? super U>> Comparator<T> thenComparing(
     Function<? super T, ? extends U> keyExtractor,
     Comparator<? super U> keyComparator);

Here, the "U extends Comparable" bound is not needed, and harmful (since 
you wouldn't be able to use this method for a non-comparable key, even 
though a Comparator is provided.)  Just a dumb cut and paste error.

Tracked at:

   https://bugs.openjdk.java.net/browse/JDK-8033590

Will be reflected in FAB library spec bundle.