x86/irq: Cleanup the arch_*_msi_irqs() leftovers

Get rid of all the gunk and remove the 'select PCI_MSI_ARCH_FALLBACK' from
the x86 Kconfig so the weak functions in the PCI core are replaced by stubs
which emit a warning, which ensures that any fail to set the irq domain
pointer results in a warning when the device is used.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112334.086003720@linutronix.de