Always remove the layer from layers array, even if session is null

[vincentfretin]

This closes aframevr/aframe#5197
You can merge and squash it with the commit 84b2da1 if you rebase -i next time you update threejs

[dmarcos]

Thanks so much. I leave this open. Working on r147 branch will be confusing and high chance for me to mess things up. I'll create a new branch and cherry pick this commit for a new super-three release.

[vincentfretin]

I see you cherry-picked the commit in https://github.com/supermedium/three.js/commits/super-147-1