powerpc/fsl-corenet: remove dead variant symbols

These are not supported as individual build targets, but instead
are supported by another target.

The dead p4040 defines in particular had bitrotted significantly.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>