Remove compatibility code for waitingSince

[t-bast]

We used to store UNIX timestamps in the waitingSince field before moving to block count. In order to ensure backward compatibility, we converted from timestamps to blockheight based on the value.

This code has shipped more than a year ago, so we can safely remove that compatibility code since it only applies during the channel open or close period, which cannot last long anyway.

Fixes #2125

[t-bast]

We've now added a serializer for block heights, so you should be able to test this.

[pm47]

Note to self: need to check after #2340 is deployed on our node.