powerpc/configs/64s: Drop SPLPAR which is default y

SPLPAR is default y since commit 20c0e826 ("powerpc/pseries:
Implement paravirt qspinlocks for SPLPAR"), so doesn't need to be in the
defconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-2-mpe@ellerman.id.au