powerpc/32: Remove #ifdef CONFIG_PPC_STD_MMU_32 in asm/book3s/32/pgtable.h

asm/book3s/32/pgtable.h is only included when CONFIG_PPC_BOOK3S_32 is set.
Whenever CONFIG_PPC_BOOK3S_32 is set, CONFIG_PPC_STD_MMU_32 is set as well.

This patch removes useless CONFIG_PPC_STD_MMU_32 #ifdefs

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>