DevTools: Remove redundant default destructor and forward declarations

Problem:
- Default destructors (and constructors) are in `.cpp` files. This
  prevents the compiler's optimizer from inlining them when it thinks
  inlining is appropriate (unless LTO is used).
- Forward declarations can prevent some optimizations, such as
  inlining of constructors and destructors.

Solution:
- Remove them or set them to `= default` and let the compiler handle
  the generation of them.
- Remove unneeded forward declarations.

Userland/DevTools/HackStudio/Project.cpp

@@ -16,10 +16,6 @@ Project::Project(const String& root_path)
     m_model = GUI::FileSystemModel::create(root_path, GUI::FileSystemModel::Mode::FilesAndDirectories);
 }
 
-Project::~Project()
-{
-}
-
 OwnPtr<Project> Project::open_with_root_path(const String& root_path)
 {
     if (!Core::File::is_directory(root_path))