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.h

@@ -19,8 +19,6 @@ class Project {
     AK_MAKE_NONMOVABLE(Project);
 
 public:
-    ~Project();
-
     static OwnPtr<Project> open_with_root_path(const String& root_path);
 
     GUI::FileSystemModel& model() { return *m_model; }