PCI: Remove unused pci_dma_burst_advice()

pci_dma_burst_advice() was added by e24c2d96 ("[PATCH] PCI: DMA
bursting advice") but apparently never used.  Remove it.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michal Simek <monstr@monstr.eu>	# microblaze
CC: David S. Miller <davem@davemloft.net>