[mc_rbdyn] Fix frame sensor update

The frames are not read in the order they are created so there's no
reason that the parent would be updated before its children frames.

src/mc_rbdyn/RobotFrame.cpp

@@ -31,8 +31,7 @@ RobotFrame::RobotFrame(NewRobotFrameToken tkn,
 
 void RobotFrame::resetForceSensor() noexcept
 {
-  if(parent_) { sensor_ = (static_cast<RobotFrame *>(parent_.get()))->sensor_; }
-  else { sensor_ = robot_.findBodyForceSensor(body()); }
+  sensor_ = robot_.findBodyForceSensor(body());
 }
 
 const std::string & RobotFrame::body() const noexcept