Fix a potential memory leak in LocalTransaction (#1120)

Author: nakamura-to

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

@@ -415,7 +415,7 @@ protected void endInternal(LocalTransactionContext context, String callerMethodN
    */
   protected void release(LocalTransactionContext context, String callerMethodName) {
     assertNotNull(context, callerMethodName);
-    localTxContextHolder.set(null);
+    localTxContextHolder.remove();
     if (!context.hasConnection()) {
       return;
     }