powerpc/nohash: Remove pgd_huge() stub

linux/hugetlb.h has a fallback pgd_huge() macro for when
pgd_huge is not defined.

Remove the powerpc redundant definitions.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ae6aa7fce84f7abcbf67f534271a4a6dd7949b0d.1662543243.git.christophe.leroy@csgroup.eu