For the moment, there's no way to remove an in-progress HTLC before
it's timed out. The other side can remove it with a routefail, but
you can't push for it to be removed.
We may add that later, but by definition it's only a polited request, and
normally we should rely on timeouts.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>