MFC r321444,r321446:

r321444:

Remove ${MTREE} and leverage etc/mtree/Makefile instead with
"make distribution".

This also fixes the fact that BSD.debug.dist was being installed if/when
${MK_DEBUG_FILES} != "no" before this commit.

r321446:

Unconditionally install etc/mtree/BSD.debug.dist again

r279248 unconditionally installed BSD.debug.dist for ease-of-developer-use.
Restore the previous behavior.

While here, add a comment to note that this is intentional to avoid accidental
future removal.

MFC with: r321444