fix: delete broken header file (#14096)

This PR deletes a `.h` file that doesn't compile.

The `util/thread.h` it imports doesn't exist

Author: eric-wieser

src/util/unlock_guard.h

@@ -1,42 +1 @@
-/*
-Copyright (c) 2013 Microsoft Corporation. All rights reserved.
-Released under Apache 2.0 license as described in the file LICENSE.
 
-Author: Leonardo de Moura
-*/
-#pragma once
-#include "util/thread.h"
-namespace lean {
-/**
-   \brief The class \c unlock_guard is a mutex wrapper that provides a
-   convenient RAII-style mechanism for releasing a mutex for the
-   duration of a scoped block.
-
-   It is the dual of lock_guard in the standard library.
-
-   Example:
-   <code>
-      {
-         lock_guard<mutex> lock(m);
-         ...
-         {
-             unlock_guard unlock(m);
-             ...
-         }
-         ...
-     }
-   </code>
-
-   \warning The calling thread must own the lock to m_mutex
-*/
-class unlock_guard {
-    mutex & m_mutex;
-public:
-    explicit unlock_guard(mutex & m):m_mutex(m) { m_mutex.unlock(); }
-    unlock_guard(unlock_guard const &) = delete;
-    unlock_guard(unlock_guard &&) = delete;
-    unlock_guard & operator=(unlock_guard const &) = delete;
-    unlock_guard & operator=(unlock_guard &&) = delete;
-    ~unlock_guard() { m_mutex.lock(); }
-};
-}