Drop non-anchor channel fee upper bound limit entirely

Drop non-anchor channel fee upper bound limit entirely

Quite a while ago we added checks for the total current dust
exposure on a channel to explicitly limit dust inflation attacks.
When we did this, we kept the existing upper bound on the channel's
feerate in place. However, these two things are redundant - the
point of the feerate upper bound is to prevent dust inflation, and
it does so in a crude way that can cause spurious force-closures.

Here we simply drop the upper bound entirely, relying on the dust
inflation limit to prevent dust inflation instead.