Remove unneccessary call to mtree, which, when building as an unprivileged
user (with -DNO_ROOT), resulted in warnings looking like these:

share/man/cat8:

user (9, 3819, not modified: Operation not permitted)
permissions (0755, 0700, modified)

The BSD.usr.mk is already taken care of in etc/Makefile.

Submitted by: Alex Richardson <alr48@cl.cam.ac.uk>
Reviewed by: bdrewery
MFC after: 1 month
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9212