powerpc/sysdev: remove unused xics_ipi_dispatch() declaration

xics_ipi_dispatch() has been removed since
commit 23d72bfd ("powerpc: Consolidate ipi message mux
and demux"), so remove it.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220913075029.682327-8-cuigaosheng1@huawei.com

arch/powerpc/include/asm/xics.h

@@ -159,7 +159,6 @@ extern void xics_setup_cpu(void);
 extern void xics_update_irq_servers(void);
 extern void xics_set_cpu_giq(unsigned int gserver, unsigned int join);
 extern void xics_mask_unknown_vec(unsigned int vec);
-extern irqreturn_t xics_ipi_dispatch(int cpu);
 extern void xics_smp_probe(void);
 extern void xics_register_ics(struct ics *ics);
 extern void xics_teardown_cpu(void);