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.h

@@ -6,7 +6,11 @@
 
 #pragma once
 
+#include "DisassemblyModel.h"
 #include "Process.h"
+#include "Profile.h"
+#include "ProfileModel.h"
+#include "SamplesModel.h"
 #include <AK/Bitmap.h>
 #include <AK/FlyString.h>
 #include <AK/JsonArray.h>
@@ -22,11 +26,6 @@
 
 namespace Profiler {
 
-class DisassemblyModel;
-class Profile;
-class ProfileModel;
-class SamplesModel;
-
 class ProfileNode : public RefCounted<ProfileNode> {
 public:
     static NonnullRefPtr<ProfileNode> create(FlyString object_name, String symbol, u32 address, u32 offset, u64 timestamp, pid_t pid)
@@ -130,7 +129,6 @@ struct ProcessFilter {
 class Profile {
 public:
     static Result<NonnullOwnPtr<Profile>, String> load_from_perfcore_file(const StringView& path);
-    ~Profile();
 
     GUI::Model& model();
     GUI::Model& samples_model();