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/MailboxTreeModel.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -15,10 +16,6 @@ MailboxTreeModel::MailboxTreeModel(AccountHolder const& account_holder)
     m_account_icon.set_bitmap_for_size(16, Gfx::Bitmap::try_load_from_file("/res/icons/16x16/home-directory.png").release_value_but_fixme_should_propagate_errors());
 }
 
-MailboxTreeModel::~MailboxTreeModel()
-{
-}
-
 GUI::ModelIndex MailboxTreeModel::index(int row, int column, GUI::ModelIndex const& parent) const
 {
     if (!parent.is_valid()) {