powerpc/powernv: Remove PCI_RESET_DELAY_US

The macro defined in arch/powerpc/platforms/powernv/pci.c isn't
used by anyone. Just remove it.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>