Transform: Don't use Automatic Constraint Plane in 2D editors

Technically it shouldn't have any effect on these editors.

The key tips in the header can be misleading.

The effect it previously had was not intended.

source/blender/editors/transform/transform.c

@@ -599,7 +599,8 @@ static bool transform_modal_item_poll(const wmOperator *op, int value)
     case TFM_MODAL_AXIS_Z:
     case TFM_MODAL_PLANE_X:
     case TFM_MODAL_PLANE_Y:
-    case TFM_MODAL_PLANE_Z: {
+    case TFM_MODAL_PLANE_Z:
+    case TFM_MODAL_AUTOCONSTRAINTPLANE: {
       if (t->flag & T_NO_CONSTRAINT) {
         return false;
       }