Disallow sending an HTLC when the balance needed is pending removal

Disallow sending an HTLC when the balance needed is pending removal

While its nice to be able to push an HTLC which spends balance that
is removed in our local commitment transaction but awaiting an RAA
from our peer for final removal its by no means a critical feature.

Because peers should really be sending RAAs quickly after we send
a commitment, this should be an exceedingly rare case, and we
already don't expose this as available balance when routing, so
this isn't even made available when sending, only forwarding.

Note that `test_pending_claimed_htlc_no_balance_underflow` is
removed as it tested a case which was only possible because of this
and now is no longer possible.