Stop including dust values in feerate affordability checks

Stop including dust values in feerate affordability checks

When we or our counterparty are updating the fees on the channel,
we currently check that the resulting balance is sufficient not
only to meet the reserve threshold, but also not push it below
dust. This isn't required in the BOLTs and may lead to spurious
force-closures (which would be a bit safer, but reserve should
always exceed the dust threshold).

Worse, the current logic is broken - it compares the output value
in *billionths of satoshis* to the dust limit in satoshis. Thus,
the code is borderline dead anyway, but can overflow for channels
with several million Bitcoin, causing the fuzzer to get mad (and
lead to spurious force-closures for few-billion-dollar channels).