vm_page: drop mpred from grab_valid_iter()

Variable mpred in vm_page_grab_valid_iter() has become a write-only
variable. Drop it.

In grab_valid_iter(), drop a pctrie_iter_reset() call already handled
in vm_page_alloc_iter().

Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D50424