Sculpt: disable undo flag for sculpt mask lasso

While this is harmless, it did cause T55399 in the past.

Sculpt adds it's own undo steps, so don't request the operator type
to do it too.

This is consistent with other sculpt operators.

source/blender/editors/sculpt_paint/paint_mask.c

@@ -535,7 +535,7 @@ void PAINT_OT_mask_lasso_gesture(wmOperatorType *ot)
 
 	ot->poll = sculpt_mode_poll;
 
-	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
+	ot->flag = OPTYPE_REGISTER;
 
 	/* properties */
 	WM_operator_properties_gesture_lasso(ot);