powerpc/cell: Drop unused iic_get_irq_host()

Sparse checking revealed that it is no longer used. The last usage was
removed in commit 2e194583

 ("[POWERPC] Cell interrupt rework") in
2006.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>