xen: Remove unnecessary BUG_ON from __unbind_from_irq()

xen: Remove unnecessary BUG_ON from __unbind_from_irq()

Commit 910f8befdf5b ("xen/pirq: fix error path cleanup when binding
MSIs") fixed a couple of errors in error cleanup path of
xen_bind_pirq_msi_to_irq(). This cleanup allowed a call to
__unbind_from_irq() with an unbound irq, which would result in
triggering the BUG_ON there.

Since there is really no reason for the BUG_ON (xen_free_irq() can
operate on unbound irqs) we can remove it.

Reported-by: Ben Hutchings <[email protected]>
Signed-off-by: Boris Ostrovsky <[email protected]>
Cc: [email protected]
Reviewed-by: Juergen Gross <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>