htlcswitch: in removeLink properly remove items from the interfaceIndex
htlcswitch: in removeLink properly remove items from the interfaceIndex

In this commit, we fix a bug in the way we handle removing items from
the interfaceIndex. Before this commit, we would delete all items items
with the target public key that of the peer that owns the link being
removed. However, this is incorrect as the peer may have other links
sill active.

In this commit, we fix this by first only deleting the link from the
peer's index, and then checking to see if the index is empty after this
deletion. Only if so do we delete the index for the peer all together.

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