chore(joinir): Phase 40-4.1.1 dead code cleanup (16 lines)

- Remove use_joinir_for_array_filter() from env.rs (9 lines)
  Route B is now default, env flag no longer needed

- Remove next_var_id() from ast_lowerer.rs (7 lines)
  Duplicate of alloc_var() method

Cumulative deletion: 51 lines (35 + 16)
Tests: 5/5 Phase 40 PASS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/frontend/ast_lowerer.rs

@@ -1038,13 +1038,6 @@ impl AstToJoinIrLowerer {
         id
     }
 
-    /// 次の変数 ID を生成
-    fn next_var_id(&mut self) -> VarId {
-        let id = crate::mir::ValueId(self.next_var_id);
-        self.next_var_id += 1;
-        id
-    }
-
     /// Phase 34-5: expr から「値を計算する JoinIR」と「結果を入れる ValueId」を返す
     ///
     /// ## 設計方針