vtd: Fix for irq bind failure after PCI attaching 32 times

vtd: Fix for irq bind failure after PCI attaching 32 times

Originally when detaching a PCI device, pirq_to_emuirq and pirq_to_irq
are freed via hypercall do_physdev_op. Now in function
pt_irq_destroy_bind_vtd, duplicated logic is added to free
pirq_to_emuirq, but not pirq_to_irq. This causes do_physdev_op fail to
free both emuirq and irq. After attaching a PCI device for 32 times,
irq resources run out. This patch removes the redundant logic.

Signed-off-by: Fengzhe Zhang <fengzhe.zhang@intel.com>