x86/msi: Remove pointless vcpu_affinity callback

Setting the irq_set_vcpu_affinity() callback to
irq_chip_set_vcpu_affinity_parent() is a pointless exercise because the
function which utilizes it searchs the domain hierarchy to find a parent
domain which has such a callback.

Remove the useless indirection.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112331.250130127@linutronix.de