remove addition of zero Z valued grid point

X.Z() never gets populated with a nonzero value besides it's initial construction value of 0. Thus this addition is extraneous.

Co-authored-by: Jonathan <jonathan.r.eastridge.civ@us.navy.mil>

usv_gazebo_plugins/src/usv_gazebo_dynamics_plugin.cc

@@ -336,7 +336,7 @@ void UsvDynamicsPlugin::Update()
       }
 
       // Vertical wave displacement.
-      double dz = depth + X.Z();
+      double dz = depth;
 
       // Total z location of boat grid point relative to water surface
       double  deltaZ = (this->waterLevel + dz) - kDdz;