Remove impl_* methods for RenderToImage and BufferedImage conversion

The following "quasi-public" methods were added in JavaFX 2.0 to support rendering to an image (snapshot) and conversion to / from a BufferedImage before public API was available:

Scene:
    renderToImage(Object)
    renderToImage(Object,float)

Image:
    impl_toExternalImage(Object)
    impl_fromExternalImage(Object)
    impl_isExternalFormatSupported(Class)

In JavaFX 2.2 we added public API to support these needs, but left the unsupported methods to allow the internal users (primarily scene builder and JDeveloper) time to migrate. In FX 8 we need to remove these methods.