Remove timedOutOutgoingHtlcs from AbstractCommitments

[akumaigorodski]

It turns out that:

• def timedOutOutgoingHtlcs(blockHeight: Long): Set[wire.UpdateAddHtlc] defined in AbstractCommitments is currently only used in Commitments object and nowhere else.
• Logic in HC commitments object can be simplified if timedOutOutgoingHtlcs takes additional arguments besides blockHeight.

So it makes sense to just remove timedOutOutgoingHtlcs from AbstractCommitments and let HC commitments object to do things differently internally without implementing of (useless) timedOutOutgoingHtlcs.

[t-bast]

Please spend more time testing those thoroughly.
Changing these abstractions creates issues for our merges to feature branches, so we need some stability on these abstractions.

[akumaigorodski]

It's safe to say this is it, the work on HC is done and I don't see a need for any more PRs (besides maybe https://gitter.im/ACINQ/developers?at=5fae6c8bc6fe0131d4f1ea88).