of/pci: Remove unused MSI controller helpers

All users of the small MSI controller API have been migrated to use the
generic MSI infrastructure instead. We no longer need a global chained list
of msi_controller. Instead, MSI controllers are now represented as IRQ
domains attached to OF nodes, and the resolution between a device
requesting an MSI and the corresponding MSI controller is done by the
generic interrupt resolution logic.

Therefore, this API is now completely useless, and can be removed from the
kernel.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rob Herring <robh@kernel.org>