powerpc/embedded6xx: Remove unused functions holly_power_off and holly_halt

Silence the following warnings triggered using W=1:

  arch/powerpc/platforms/embedded6xx/holly.c:236:6: error: no previous prototype for 'holly_power_off'
  arch/powerpc/platforms/embedded6xx/holly.c:243:6: error: no previous prototype for 'holly_halt'

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>