PCI: remove pci_enable_msix

Unused now that all callers switched to pci_alloc_irq_vectors.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>