powerpc: Remove unused CPU_FTR_ARCH_201

The last usage was removed in c17b98cf ("KVM: PPC: Book3S HV:
Remove code for PPC970 processors") (Dec 2014).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>