refactor: unbindResource 메서드는 반환값이 존재하지 않는다.

jdbc/src/main/java/org/springframework/transaction/support/TransactionSynchronizationManager.java

@@ -28,9 +28,9 @@ public static void bindResource(DataSource key, Connection value) {
         resource.put(key, value);
     }
 
-    public static Connection unbindResource(DataSource key) {
+    public static void unbindResource(DataSource key) {
         final Map<DataSource, Connection> resource = resources.get();
-        return resource.remove(key);
+        resource.remove(key);
     }
 
 }

jdbc/src/test/java/org/springframework/transaction/support/TransactionSynchronizationManagerTest.java

@@ -0,0 +1,6 @@
+package org.springframework.transaction.support;
+
+class TransactionSynchronizationManagerTest {
+
+
+}