powerpc/iommu: Remove unused pci_iommu_init() and pci_direct_iommu_init()

pci_iommu_init() and pci_direct_iommu_init() are not referenced anywhere,
so remove them.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>