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/Mail/MailWidget.cpp

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
  * Copyright (c) 2021, Undefine <cqundefine@gmail.com>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -95,10 +96,6 @@ MailWidget::MailWidget()
     };
 }
 
-MailWidget::~MailWidget()
-{
-}
-
 bool MailWidget::connect_and_login()
 {
     auto server = Config::read_string("Mail", "Connection", "Server", {});