x86: Remove never-called arch_setup_msi_irq()

Since commit 75c46fa6

, "x64, x2apic/intr-remap: MSI and MSI-X
support for interrupt remapping infrastructure", x86 has had an
implementation of arch_setup_msi_irqs().

That implementation does not call arch_setup_msi_irq(), instead it calls
setup_irq(). No other x86 code calls arch_setup_msi_irq().

That leaves only arch_setup_msi_irqs() in drivers/pci/msi.c, but that
routine is overridden by the x86 version of arch_setup_msi_irqs().

So arch_setup_msi_irq() is dead code, remove it.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>