powerpc/64: get rid of MIN_HUGEPTE_SHIFT

MIN_HUGEPTE_SHIFT hasn't been used since commit d1837cba
("powerpc/mm: Cleanup initialization of hugepages on powerpc")

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