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/Profiler/Profile.cpp

@@ -47,10 +47,6 @@ Profile::Profile(Vector<Process> processes, Vector<Event> events)
     rebuild_tree();
 }
 
-Profile::~Profile()
-{
-}
-
 GUI::Model& Profile::model()
 {
     return *m_model;