Blender Git Loki

Cleanup: remove Cycles layer bits checking in the kernel.

At some point the idea was that we could have an optimization where we could
render multiple render layers without re-exporting the scene, by just updating
the layer bits. We are not doing this now and in practice with the available
render layer control like exclude layers it's not always possible anyway.

This makes it easier to support an arbitrary number of layers in the future
(hopefully this summer), and frees up some useful bits in the kernel.

Reviewed By: sergey, dingto

Differential Revision: https://developer.blender.org/D2020