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/ProcessFileDescriptorMapWidget.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -41,10 +42,6 @@ ProcessFileDescriptorMapWidget::ProcessFileDescriptorMapWidget()
     m_table_view->set_model(MUST(GUI::SortingProxyModel::create(*m_model)));
 }
 
-ProcessFileDescriptorMapWidget::~ProcessFileDescriptorMapWidget()
-{
-}
-
 void ProcessFileDescriptorMapWidget::set_pid(pid_t pid)
 {
     if (m_pid == pid)