htlcswitch: fix duplicate close

The decayed log database opening and closing is managed at a higher
level in config_builder.go.

htlcswitch/decayedlog.go

@@ -158,9 +158,6 @@ func (d *DecayedLog) Stop() error {
 
 	d.wg.Wait()
 
-	// Close boltdb.
-	d.db.Close()
-
 	return nil
 }