powerpc/powermac: Remove stale declaration of pmac_md

pmac_md doesn't exist anymore, remove stall declaration.

Fixes: e8222502ee ("[PATCH] powerpc: Kill _machine and hard-coded platform numbers")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b2e52934e5a500f149e6d94db3cfa0569bc35081.1630657402.git.christophe.leroy@csgroup.eu

arch/powerpc/platforms/powermac/setup.c

@@ -79,8 +79,6 @@ int pmac_newworld;
 
 static int current_root_goodness = -1;
 
-extern struct machdep_calls pmac_md;
-
 #define DEFAULT_ROOT_DEVICE Root_SDA1	/* sda1 - slightly silly choice */
 
 #ifdef CONFIG_PPC64