Remove wall objects body

[FelipeMartins96]

As per described on the ODE manual on how to create an immovable object:
http://ode.org/wiki/index.php?title=Manual#Geoms

How can an immovable body be created?

In other words, how can you create a body that doesn't move, but that interacts with other bodies? The answer is to create a geom only, without the corresponding rigid body object. The geom is associated with a rigid body ID of zero. Then in the contact callback when you detect a collision between two geoms with a nonzero body ID and a zero body ID, you can simply pass those two IDs to the dJointAttach function as normal. This will create a contact between the rigid body and the static environment.

Don't try to get the same effect by setting a very high mass/inertia on the "motionless" body and then resetting it's position/orientation on each time step. This can cause unexpected simulation errors.

To create an immovable object we should define it only as a geometry, currently, we also create a body for each wall. This should be tested for performance improvements