PCI: Remove unused pci_vpd_truncate()

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 db567943

 ("PCI: add interface to set visible size of
VPD"), removing this interface:

    pci_vpd_truncate()

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