powerpc/powermac: Remove stale declaration of pmac_md

pmac_md doesn't exist anymore, remove stall declaration.

Fixes: e8222502 ("[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