vm_page: drop page_iter_lookup

The functions vm_page_iter_lookup and vm_page_iter_lookup_ge are just
wrappers around vm_radix_iter_lookup and vm_radix_iter_lookup_ge,
respectively. They server no real purpose, so drop them and use the
vm_radix versions everywhere.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49203