PCI: Remove unused pci_enable_device_io()

After the last user was removed, remove this PCI core function.  It's very
unlikely that we'll see a new device requiring io space access, even though
memory space access is supported.

Link: https://lore.kernel.org/r/213ebf62-53a3-42b7-8518-ecd5cd6d6b08@gmail.com
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>