PCI/portdrv: Remove pcie_portdrv_err_handler.slot_reset

The pci_error_handlers.slot_reset() callback is only used for non-bridge
devices (see broadcast_error_message()).  Since portdrv only binds to
bridges, we don't need pcie_portdrv_slot_reset(), so remove it.

Signed-off-by: Oza Pawandeep <poza@codeaurora.org>
[bhelgaas: changelog, remove pcie_portdrv_slot_reset() completely]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>