avoid attribute ambiguity in ExprStmt

Both expr stmt and expr can have attributes, which doesn't make sense.
Let's say that statement's attrs are those of the expression.
diff --git a/rust.ungram b/rust.ungram
index 52bdf32..5a8e116 100644
--- a/rust.ungram
+++ b/rust.ungram
@@ -318,7 +318,7 @@
   '=' initializer:Expr ';'
 
 ExprStmt =
-  Attr* Expr ';'?
+  Expr ';'?
 
 Expr =
   ArrayExpr