powerpc/book3e: drop mmu_get_tsize()

This function is not used anymore, drop it.

Fixes: b42279f0 ("powerpc/mm/nohash: MM_SLICE is only used by book3s 64")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>