powerpc: remove unused udbg_init_debug_beat() declaration

udbg_init_debug_beat() has been removed since
commit bf4981a0

 ("powerpc: Remove the celleb support"),
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/20220913075029.682327-5-cuigaosheng1@huawei.com