powerpc/ptrace: drop PARAMETER_SAVE_AREA_OFFSET

PARAMETER_SAVE_AREA_OFFSET is not used, drop it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6dac2b49207647f75cbf0e6771a545e691f0fd93.1582848567.git.christophe.leroy@c-s.fr