This was only being used in one place in compose_closed_ui
to create the label for the closed composebox. But it
only checked if the `channel` and `topic` filters existed,
while `stream_sub` can return `undefined` for a few other
reasons. To ensure that we're catching the undefined sub
while also avoiding duplicate work, it makes sense to just
call `stream_sub()` directly.
Fixes this bug:
https://zulip.sentry.io/issues/5367251929/events/40073ecf007a4a9798e728061a576377/?project=450455688282112