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

share/examples/Makefile

@@ -240,8 +240,6 @@ copies:
 		rm -f ${DESTDIR}${BINDIR}/$i; \
 	fi
 .endfor
-	mtree -deU ${MTREE_FOLLOWS_SYMLINKS} \
-	    -f ${SRCTOP}/etc/mtree/BSD.usr.dist -p ${DESTDIR}/usr
 .for file in ${XFILES}
 	${INSTALL} -T package=${PACKAGE_${file}:Uexamples} -o ${SHAREOWN} -g ${SHAREGRP} -m ${SHAREMODE} \
 		${.CURDIR}/${file} ${DESTDIR}${BINDIR}/${file}