wallet: only remove conflicting unmined transactions
wallet: only remove conflicting unmined transactions

In this commit, we fix a bug introduced in an earlier commit. Before
this commit, we would *always* remove an unmined transaction if it
failed to be accepted by the network upon restart. Instead, we should
only remove transaction that are actually due to us trying to spend an
output that’s already spent, or an orphan transaction.

btcsuite/btcwallet,btcsuite/btcwallet