vm_page_replace: remove redundant radix lookup

Remove redundant lookup of the old page from vm_page_replace. Verification
that the old page exists is already done by vm_radix_replace.

Submitted by: Ryan Libby <rlibby@gmail.com>
Reviewed by: alc, kib
Sponsored by: EMC / Isilon Storage Division
Follow-up to: https://reviews.freebsd.org/D4326
Differential Revision: https://reviews.freebsd.org/D4471