multi: make decayed log DB remote compatible

Even though the sphinx router's persistent replay log is not crucial in
the operation of lnd as its state can be re-created by creating a new
brontide connection, we want to make lnd fully stateless and therefore
have the option of not storing any state on disk.

config.go

@@ -203,8 +203,6 @@ var (
 	bitcoindEstimateModes       = [2]string{"ECONOMICAL", defaultBitcoindEstimateMode}
 
 	defaultPrunedNodeMaxPeers = 4
-
-	defaultSphinxDbName = "sphinxreplay.db"
 )
 
 // Config defines the configuration options for lnd.