Blender Git Loki

Collections/Layer cleanup: Remove TODO_LAYER_OVERRIDE from most places

This was originally a good idea. However we will need to pay special attention
to this when doing the dynamic overrides anyways. The placeholders won't be
enough to spare us that job.

That said I left the ones on layer.c because we are actually calling these
BKE_override_*_add() functions from doversion, yet they don't do anything.