Depsgraph: Remove bmain from graph

It really should not care about bmain.

source/blender/depsgraph/intern/builder/deg_builder_relations_scene.cc

@@ -75,7 +75,6 @@ void DepsgraphRelationBuilder::build_scene(Main *bmain, Scene *scene)
 	}
 
 	/* XXX store scene to access from DAG_get_scene */
-	m_graph->bmain = bmain;
 	m_graph->scene = scene;
 
 	/* scene objects */

source/blender/depsgraph/intern/depsgraph.h

@@ -166,7 +166,6 @@ struct Depsgraph {
 	SpinLock lock;
 
 	// XXX: additional stuff like eval contexts, mempools for allocating nodes from, etc.
-	Main *bmain;  /* XXX: For until depsgraph has proper ownership. */
 	Scene *scene; /* XXX: We really shouldn't do that, but it's required for shader preview. */
 };