lnwallet: eliminate usage of LightningChannel.theirPrevPkScript
lnwallet: eliminate usage of LightningChannel.theirPrevPkScript

This commit removes the theirPrevPkScript field from the
LightningChannel struct all together. It’s no longer needed as the more
fundamental mutation bug has been fixed within the channel state
machine.

aakselrod/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,LightningNetwork/lnd,ddustin/lnd