Applications: Use default constructors/destructors

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Applications/SystemMonitor/ThreadStackWidget.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -73,10 +74,6 @@ ThreadStackWidget::ThreadStackWidget()
     m_stack_table->set_model(adopt_ref(*new ThreadStackModel()));
 }
 
-ThreadStackWidget::~ThreadStackWidget()
-{
-}
-
 void ThreadStackWidget::show_event(GUI::ShowEvent&)
 {
     refresh();