Add voicings of m6/9 and m6/♭9 with no 5th.

This clears up a FIXME comment suggesting that we were missing some
chords with perfect 5ths. The comment was somewhat incorrect. One
chord was present all along. One should have been present, but was
incorrect (until the last commit). The third was legitimately missing
(until the last commit).

Now that we have all of these in place with perfect 5ths, we can also
add the versions voiced with no 5th at all and delete the comment.