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