Replace `send_htlc` amount checking with available balances

Replace `send_htlc` amount checking with available balances

Now that the `get_available_balances` min/max bounds are exact, we
can stop doing all the explicit checks in `send_htlc` entirely,
instead comparing against the `get_available_balances` bounds and
failing if the amount is out of those bounds.

This breaks support for sending amounts below the dust limit if
there is some amount of dust exposure remaining before we hit our
cap, however we will no longer generate such routes anyway.