vm_page: remove obsolete page alloc loops

When page_alloc_noobj functions were introduced, the need for loops to
support cycles of try-alloc, then wait, then retry disppeared from
vm_page_alloc_domain and vm_page_alloc_contig_domain functions, but
loops remained. Remove them.

Reported by: alc
Reviewed by: alc, markj, kib
Differential Revision: https://reviews.freebsd.org/D50263