vm_domainset: Simplify vm_domainset_iter_next()

As we are now visiting each domain only once, the test in
vm_domainset_iter_prefer() about skipping the preferred domain (the one
initially visited for policy DOMAINSET_POLICY_PREFER) becomes redundant.
Removing it makes this function essentially the same as
vm_domainset_iter_rr().

Thus, remove vm_domainset_iter_prefer(). This makes all policies behave
the same in vm_domainset_iter_next().

No functional change (intended).

PR: 277476
MFC after: 10 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51250