powerpc/pci: Delete traverse_pci_dn()

Nothing uses it.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200902035121.1762475-1-oohall@gmail.com