powerpc/powernv: Remove pnv_pci_probe_mode()

The callback (ppc_md.pci_probe_mode()) is used to determine if the
child PCI devices of the indicated PCI bus should be probed from
device-tree or hardware. On PowerNV platform, we always expect
probing PCI devices from hardware, which is PowerPC PCI core's
default behaviour. Also, the callback had some delay implemented
based on PHB's device node property "reset-clear-timestamp", which
wasn't exported from skiboot. So we don't need this function and
it's safe to remove it.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>