powerpc: remove unused chrp_event_scan() declaration

chrp_event_scan() has been removed since
commit 3d541c4b ("powerpc/chrp: Use the same RTAS
daemon as pSeries"), 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/20220913075029.682327-4-cuigaosheng1@huawei.com