Implicit prefinalizer registration.
Switch to implicit registration of prefinalizers along with removing
the ability to dynamically unregister a prefinalizer; the latter
being an unused feature.
The requirement to manually register a prefinalizer has proven to be
a chore and a source of bugs. Case in point: HTMLCanvasElement
currently declares a prefinalizer, but doesn't register it. Simplify
the programming model by automatically registering prefinalizers.