powerpc/ipic: Remove unused ipic_set_priority()

ipic_set_priority() has been unused since 2006 when the last usage was
removed in commit b9f0f1bb ("[POWERPC] Adapt ipic driver to new
host_ops interface, add set_irq_type to set IRQ sense").

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>