Kernel: Remove no-longer-used Lockable template

Kernel/Locking/Lockable.h

@@ -1,35 +0,0 @@
-/*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#pragma once
-
-#include <Kernel/Locking/Mutex.h>
-
-namespace Kernel {
-
-template<typename T>
-class Lockable {
-public:
-    Lockable() = default;
-    Lockable(T&& resource)
-        : m_resource(move(resource))
-    {
-    }
-    [[nodiscard]] Mutex& lock() { return m_lock; }
-    [[nodiscard]] T& resource() { return m_resource; }
-
-    [[nodiscard]] T lock_and_copy()
-    {
-        MutexLocker locker(m_lock);
-        return m_resource;
-    }
-
-private:
-    T m_resource;
-    Mutex m_lock;
-};
-
-}