chainntnfs/neutrinonotify: remove old spend notification handling logic
chainntnfs/neutrinonotify: remove old spend notification handling logic

In this commit, we remove the old spend notification logic within the
NeutrinoNotifier as it's been phased out by the TxNotifier.

aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd