share/examples: use mtree -W flag when installing with -DNO_ROOT

[arichardson]

Summary

The call to mtree should not be necessary as the directories should already
have been created.

This removes thousands of lines of warning output from make installworld
with -DNO_ROOT:

.: user (0, 3375, not modified: Operation not permitted)

gid (0, 3375, not modified: Operation not permitted)

share: user (0, 3375, not modified: Operation not permitted)

gid (0, 3375, not modified: Operation not permitted)

share/misc:

user (0, 3375, not modified: Operation not permitted)
gid (0, 3375, not modified: Operation not permitted)

etc...

[brooks]

This seems pretty reasonable. I'm a little leary of not setting modes, but in practice that's probably ok for NO_ROOT installs.

[trasz]

I wonder if the line should be removed instead. BSD.usr.dist seems to already be handled in etc/Makefile.

The line was introduced in r100346 by ru@, and later updated (just the mechanical path update) in r314652 by ngie@.

[arichardson]

Removed the mtree invocation instead