PNPBIOS: remove unused pnpid32_to_pnpid() declaration

All uses of pnpid32_to_pnpid() were removed by
commit 25eb8461

 ("PNP: add pnp_eisa_id_to_string()"),
so remove the declaration, too.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>