classes/image: drop unused default of MACHINE_POSTPROCESS_COMMAND

In the daisy (1.6) timeframe, when we rewrote the image construction in
Python, we neglected to reimplement the support for the little used and
undocumented variable MACHINE_POSTPROCESS_COMMAND, and apparently nobody
noticed. We have a better method for implementing machine-specific image
formats that is in wider use (i.e. add a custom class which implements
the new image type, add the class to IMAGE_CLASSES and the type to
IMAGE_FSTYPES), and we now also have wic. Thus it makes more sense to
just call this variable unsupported now and drop the sole remaining
reference to it.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>