bugfix - do not update the deleted items on env.step()

[askuric]

Hi @jhavl,

This is a quick PR. I've stumbled upon a problem when I add a mesh file to the environment and remove it later on.

...
env.add(poly_mesh)
...
env.remove(poly_mesh)
...
env.step()

basically the for loop inisde the env.step() does not check if the object has been deleted or not. So I've added it.
With this change the above code work properly.

[KuabeM]

Stumbled upon the same issue and tested this fix. Works for me!