Clean up dead tunnel code

Running go vet on tunnel package returns:
$ go vet ./...
./default.go:929:2: unreachable code
./link.go:104:2: unreachable code
./listener.go:184:2: unreachable code
./session.go:241:2: unreachable code

tunnel/default.go

@@ -919,14 +919,12 @@ func (t *tun) Close() error {
 	default:
 		close(t.closed)
 		t.connected = false
+	}
 
 	// send a close message
 	// we don't close the link
 	// just the tunnel
 	return t.close()
-	}
-
-	return nil
 }
 
 // Dial an address

tunnel/link.go

@@ -98,7 +98,6 @@ func (l *link) Close() error {
 		return nil
 	default:
 		close(l.closed)
-		return nil
 	}
 
 	return nil

tunnel/listener.go

@@ -181,5 +181,4 @@ func (t *tunListener) Accept() (Session, error) {
 		}
 		return c, nil
 	}
-	return nil, nil
 }

tunnel/session.go

@@ -237,8 +237,6 @@ func (s *session) Send(m *transport.Message) error {
 	case <-s.closed:
 		return io.EOF
 	}
-
-	return nil
 }
 
 // Recv is used to receive a message