PCI/portdrv: Remove unused pcie_port_bus_{,un}register() declarations

Commit c6c889d9 ("PCI/portdrv: Remove pcie_port_bus_type link order
dependency") removed pcie_port_bus_{,un}register() but erroneously
retained their declarations in portdrv.h.  Remove them as well.

Link: https://lore.kernel.org/r/7fd76b0591c37287ab94d911d8fd9ab9a2afcd16.1627638184.git.lukas@wunner.de


Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>