src: remove out-of-date TODO comment

This commit attempts to fix one of the items in
https://github.com/nodejs/node/issues/4641, which was to remove a
TODO the UDPWrap::OnSend function and share the code in that method with
StreamWrap::AfterWrite.

While looking into this addaleax pointed out that the implementations
for these two functions have diverged since the original comment
was added:

$ git log --pretty=short -u -L 357,357:src/udp_wrap.cc

$ git show cbd40336:src/udp_wrap.cc |
grep -1 -A26 'UDPWrap::OnSend'

git show cbd40336:src/stream_wrap.cc |
grep -A27 'void StreamWrap::AfterWrite'

Removing the TODO comment seems appropriate in this case.

PR-URL: https://github.com/nodejs/node/pull/9000


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>