vm: Retire vm_page_alloc_freelist{,_domain}()

Once upon a time, I created vm_page_alloc_freelist{,_domain}() to
support faster allocation of pages that were mapped by the partial
direct map on 32-bit MIPS. At the time, I expected that these
functions might find other uses too, but those other uses never
materialized. So, these functions have not been used for some time
now. Instead, people use the more general vm_page_alloc_contig().

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D46063