[POWERPC] spufs: remove SPU_CONTEXT_PREEMPT

Remove the SPU_CONTEXT_PREEMPT define.  It's unused and won't be used
in this form after the scheduler rework.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>