kill perpetually-useless TODO doc

In practice the things that accumulate in this TODO file (and
probably many others) are some combination of:

(a) obvious
(b) irrelevant
(c) outdated
(d) already filed as an actual feature request issue
(e) so far out that it's pointless to contemplate them
   until you've actually committed to doing them in the
   near future.

At the end of the day, the file isn't very useful.