lightning-liquidity: Eventually drop peers from the ignore list

[tnull]

In lightningdevkit/lightning-liquidity#116 we introduced the notion of a ignore list to which peers are added that send us bogus data (rather than disconnecting them, which could result in also disconnecting any operational channels).

While the list is currently not persisted, we eventually will, at which point it will be all the more necessary to give peers a second chance after a certain time interval.

[tnull]

This was done in #3436: we now simply remove the entries on peere disconnection.