Drop the hunt_channel_id from the metadata passed in the puzzle modal

Drop the hunt_channel_id from the metadata passed in the puzzle modal

We've now got another handoff, from the puzzle modal's submission
handler to a separate channel_created event before the hunt's
channel_id is needed. And by then we've lost the context of this
private_metadata, (so the current code scans through the database
'hunts' table to find the ID). Given all that, we can drop this unused
field from the metadata.