Followup for 1728c1e: we don't need to replace flags at all

Replacing the flags isn't thread safe and could lead to bi troubles.
Such things are to be handled by the dependency graph.

source/blender/blenkernel/intern/depsgraph.c

@@ -690,6 +690,8 @@ static void build_dag_object(DagForest *dag, DagNode *scenenode, Scene *scene, O
 			if (ob->type == OB_FONT) {
 				if (cu->textoncurve) {
 					node2 = dag_get_node(dag, cu->textoncurve);
+					/* Text on curve requires path to be evaluated for the target curve. */
+					node2->eval_flags |= DAG_EVAL_NEED_CURVE_PATH;
 					dag_add_relation(dag, node2, node, DAG_RL_DATA_DATA | DAG_RL_OB_DATA, "Texture On Curve");
 				}
 			}

source/blender/blenkernel/intern/font.c

@@ -865,11 +865,6 @@ makebreak:
 	/* TEXT ON CURVE */
 	/* Note: Only OB_CURVE objects could have a path  */
 	if (cu->textoncurve && cu->textoncurve->type == OB_CURVE) {
-		Curve *cucu = cu->textoncurve->data;
-		int oldflag = cucu->flag;
-		
-		cucu->flag |= (CU_PATH + CU_FOLLOW);
-
 		BLI_assert(cu->textoncurve->curve_cache != NULL);
 		if (cu->textoncurve->curve_cache->path) {
 			float distfac, imat[4][4], imat3[3][3], cmat[3][3];
@@ -967,7 +962,6 @@ makebreak:
 				
 			}
 		}
-		cucu->flag = oldflag;
 	}
 
 	if (selboxes) {