powerpc/mm: Move vma_mmu_pagesize()

vma_mmu_pagesize() is only required for slices,
otherwise there is a generic weak version doing the
exact same thing.

Move it to slice.c

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1302e000d529c93d07208f1fae90f938e7a551b4.1649523076.git.christophe.leroy@csgroup.eu