PCI/MSI: Remove unused pcibios_msi_controller() hook

The pcibios_msi_controller() hook was only implemented by ARM, and it sets
pci_bus->msi now, so it doesn't need this hook anymore.

Remove the unused pcibios_msi_controller() hook.

[bhelgaas: changelog, split into separate patch]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>