xen/pci: Remove 'xen_allocate_pirq_gsi'.

xen/pci: Remove 'xen_allocate_pirq_gsi'.

In the past (2.6.38) the 'xen_allocate_pirq_gsi' would allocate
an entry in a Linux IRQ -> {XEN_IRQ, type, event, ..} array. All
of that has been removed in 2.6.39 and the Xen IRQ subsystem uses
an linked list that is populated when the call to
'xen_allocate_irq_gsi' (universally done from any of the xen_bind_*
calls) is done. The 'xen_allocate_pirq_gsi' is a NOP and there is
no need for it anymore so lets remove it.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>