DevTools: 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/DevTools/HackStudio/Git/DiffViewer.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -16,10 +17,6 @@
 
 namespace HackStudio {
 
-DiffViewer::~DiffViewer()
-{
-}
-
 void DiffViewer::paint_event(GUI::PaintEvent& event)
 {
     GUI::Painter painter(*this);