PCI/PM: Remove unused pcibios_pm_ops

PCI/PM: Remove unused pcibios_pm_ops

The "struct dev_pm_ops pcibios_pm_ops", declared in include/linux/pci.h and
defined in drivers/pci/pci-driver.c, provided arch-specific hooks when a
PCI device was doing a hibernate transition.

394216275c7d ("s390: remove broken hibernate / power management support")
removed the last use of pcibios_pm_ops, so remove it completely.

[bhelgaas: drop unused "error"]
Link: https://lore.kernel.org/r/20200730194416.1029509-1-vaibhavgupta40@gmail.com
Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>