powerpc/xmon: remove unused ppc_parse_cpu() declaration

ppc_parse_cpu() has been removed since
commit 5b102782 ("powerpc/xmon: Enable disassembly
files (compilation changes)"), 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/20220913075029.682327-2-cuigaosheng1@huawei.com