[POWERPC] remove unused io accessors

The io accessors insw_ns, outsw_ns, insl_ns and outsl_ns are unused
(except for one unnecessary use in drivers/net/3c509.c that is addressed
in a previous patch) and are only defined in powerpc/ppc, so remove them.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>