Kernel: Use RefPtr instead of LockRefPtr for File and subclasses

This was mostly straightforward, as all the storage locations are
guarded by some related mutex.

The use of old-school associated mutexes instead of MutexProtected
is unfortunate, but the process to modernize such code is ongoing.

Kernel/FileSystem/FIFO.h

@@ -24,7 +24,7 @@ public:
         Writer
     };
 
-    static ErrorOr<NonnullLockRefPtr<FIFO>> try_create(UserID);
+    static ErrorOr<NonnullRefPtr<FIFO>> try_create(UserID);
     virtual ~FIFO() override;
 
     UserID uid() const { return m_uid; }