PCI: Remove unused Optimized Buffer Flush/Fill support

My philosophy is unused code is dead code.  And dead code is subject to bit
rot and is a likely source of bugs.  Use it or lose it.

This reverts 48a92a81

 ("PCI: add OBFF enable/disable support"),
removing these interfaces:

    pci_enable_obff()
    pci_disable_obff()

[bhelgaas: split to separate patch, also remove prototypes from pci.h]
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>