Actually remove old event listeners

[MonoidMusician]

eventListener creates a new reference (that's why it's in Effect), distinct from old ones, so we need to store the result of that instead of the callback we passed to it

[MonoidMusician]

Example to reproduce it with:

D.button
  ((D.OnClick := _) <<< (log :: String -> Effect Unit) <$>
    oneOfMap bang ["You should", "only see", "this last one"]
  )
  [ text_ "Useless machine" ]

[mikesol]

Thanks @MonoidMusician !