Blender Git Loki

Fix renderlayers UI removing the wrong renderlayer

Bug introduced on 1c4c28872721 (well technically in b48694639a).
We should not remove the renderlayer from the context, but instead the one that
is active from scene.

That said, the UI should make a distinction between the scene active render layer
and the one that is active in the UI (and that should be the one used when
removing it).

But for now this is at least more consistent for the users.