Libraries: Use default constructors/destructors in LibGUI

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/Libraries/LibGUI/FileSystemModel.cpp

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021, sin-ack <sin-ack@protonmail.com>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -271,10 +272,6 @@ FileSystemModel::FileSystemModel(String root_path, Mode mode)
     invalidate();
 }
 
-FileSystemModel::~FileSystemModel()
-{
-}
-
 String FileSystemModel::name_for_uid(uid_t uid) const
 {
     auto it = m_user_names.find(uid);