flutter: 去掉 position 中冗余的 rule 成员变量

src/ui/flutter/lib/mill/position.dart

@@ -40,8 +40,6 @@ class StateInfo {
 }
 
 class Position {
-  Rule rule;
-
   GameResult result = GameResult.pending;
 
   List<String> board = List<String>(sqNumber);
@@ -417,8 +415,6 @@ class Position {
   int setPosition(Rule newRule) {
     result = GameResult.pending;
 
-    rule = new Rule(); // TODO
-
     gamePly = 0;
     rule50 = 0;
     pliesFromNull = 0;