Depsgraph: Remove meaningless evaluation mode

With the current implementation it only confuses logic around
checks like vewport/render subdivision levels.

If this mode is really needed for any decision making, implement
this properly.

source/blender/depsgraph/DEG_depsgraph.h

@@ -71,7 +71,6 @@ struct ViewLayer;
 typedef enum eEvaluationMode {
 	DAG_EVAL_VIEWPORT       = 0,    /* evaluate for OpenGL viewport */
 	DAG_EVAL_RENDER         = 1,    /* evaluate for render purposes */
-	DAG_EVAL_BACKGROUND     = 2,    /* evaluate in background for baking/caching */
 } eEvaluationMode;
 
 /* DagNode->eval_flags */

source/blender/depsgraph/intern/depsgraph_query_filter.cc

@@ -200,7 +200,7 @@ Depsgraph *DEG_graph_filter(const Depsgraph *graph_src, Main *bmain, DEG_FilterQ
 	/* TODO: Improve the builders to not add any ID nodes we don't need later (e.g. ProxyBuilder?) */
 	Depsgraph *graph_new = DEG_graph_new(deg_graph_src->scene,
 	                                     deg_graph_src->view_layer,
-	                                     DAG_EVAL_BACKGROUND);
+	                                     deg_graph_src->mode);
 	DEG_graph_build_from_view_layer(graph_new,
 	                                bmain,
 	                                deg_graph_src->scene,