powerpc: Remove some unused functions

Remove slice_set_psize() which is not used.

It was added in 3a8247cc "powerpc: Only demote individual slices
rather than whole process" but was never used.

Remove vsx_assist_exception() which is not used.

It was added in ce48b210 "powerpc: Add VSX context save/restore,
ptrace and signal support" but was never used.

Remove generic_mach_cpu_die() which is not used.

Its last caller was removed in 375f561a "powerpc/powernv: Always go
into nap mode when CPU is offline".

Remove mpc7448_hpc2_power_off() and mpc7448_hpc2_halt() which are
unused.

These were introduced in c5d56332 "[POWERPC] Add general support for
mpc7448hpc2 (Taiga) platform" but were never used.

This was partially found by using a static code analysis program called
cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
[mpe: Update changelog with details on when/why they are unused]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>