powerpc/ps3: Fix build error on UP

A non-SMP version of smp_send_stop() is now included in smp.h.
Remove the unneeded definition in the PS3 smp.c.

Fixes build errors like these when CONFIG_SMP=n:

  arch/powerpc/platforms/ps3/setup.c:49: error: redefinition of 'smp_send_stop'
  include/linux/smp.h:125: error: previous definition of 'smp_send_stop' was here

Reported-by: Subrata Modak <subrata@linux.vnet.ibm.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>