Fix a potential memory leak in LocalTransaction (#1122)

Author: nakamura-to

doma-core/src/main/java/org/seasar/doma/jdbc/tx/LocalTransaction.java

@@ -216,7 +216,7 @@ public LocalTransactionContext suspend() {
     if (!isActiveInternal(context)) {
       throw new TransactionNotYetBegunException(Message.DOMA2046);
     }
-    localTxContextHolder.set(null);
+    localTxContextHolder.remove();
     return context;
   }