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/ProcessStateWidget.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -89,7 +90,3 @@ ProcessStateWidget::ProcessStateWidget(pid_t pid)
     m_table_view->column_header().set_visible(false);
     m_table_view->column_header().set_section_size(0, 90);
 }
-
-ProcessStateWidget::~ProcessStateWidget()
-{
-}