markdown: Remove obsolete dependency on util.js.
markdown: Remove obsolete dependency on util.js.

We are gonna phase out util.get_message_topic()
in our entire codebase eventually, but we
certainly don't need it here, since the local
echo codepath is using brand new objects that
we construct inside the compose code, and
there's no danger of legacy "subject" data.

My goal for the markdown code is to keep it
free of any accidental dependencies that we
can easily avoid, as I think there's some
possible future where we split out the code
as its own library for people who want to
render Zulip markdown in non-core projects.

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