vm_page: Drop handling of VM_ALLOC_NOOBJ in vm_page_alloc_contig_domain()

As in vm_page_alloc_domain_after(), unconditionally preserve PG_ZERO.

Implement vm_page_alloc_noobj_contig_domain().

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32034