vm_map: drop vm_map_simplify_entry.9

A change in 2019 removed the function vm_map_simplify_entry(), but failed
to remove the manual page that documents it. As that was an internal
function not meant to be used outside the kernel, the manual page is
dropped, and a new manual page for the function that replaced it is
not added.

Reported by: pfg
Reviewed by: alc
Fixes: 83ea714f4fca ("vm_map_simplify_entry considers merging")
Differential Revision: https://reviews.freebsd.org/D50861