powerpc/64s: Remove ICSWX feature from Power9

Power9 does not implement the icswx instruction. This CPU feature is not visible
to userspace and is only used in the CONFIG_PPC_ICSWX code, which is generally
not enabled, and can only be triggered by other code using icswx, which should
not happen on Power9 systems in the first place. So impact should be minimal.

Fixes: c3ab300e ("powerpc: Add POWER9 cputable entry")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>