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

arch/powerpc/include/asm/udbg.h

@@ -42,7 +42,6 @@ extern void __init udbg_init_maple_realmode(void);
 extern void __init udbg_init_pas_realmode(void);
 extern void __init udbg_init_rtas_panel(void);
 extern void __init udbg_init_rtas_console(void);
-extern void __init udbg_init_debug_beat(void);
 extern void __init udbg_init_btext(void);
 extern void __init udbg_init_44x_as1(void);
 extern void __init udbg_init_40x_realmode(void);