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/OpenFileDescription.h

@@ -141,7 +141,7 @@ private:
     }
 
     RefPtr<Inode> m_inode;
-    NonnullLockRefPtr<File> m_file;
+    NonnullRefPtr<File> m_file;
 
     struct State {
         OwnPtr<OpenFileDescriptionData> data;