powerpc: get rid of PMD_PAGE_SIZE() and _PMD_SIZE

PMD_PAGE_SIZE() is nowhere used and _PMD_SIZE is only
used by PMD_PAGE_SIZE().

This patch removes them.

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