Revert "util: fix clang-tidy modernize-use-equals-default error"

This reverts commit 753d2b1.

Author: hebasto

include/mp/util.h

@@ -170,7 +170,7 @@ class MP_CAPABILITY("mutex") Mutex {
 class MP_SCOPED_CAPABILITY Lock {
 public:
     explicit Lock(Mutex& m) MP_ACQUIRE(m) : m_lock(m.m_mutex) {}
-    ~Lock() MP_RELEASE() = default;
+    ~Lock() MP_RELEASE() {}
     void unlock() MP_RELEASE() { m_lock.unlock(); }
     void lock() MP_ACQUIRE() { m_lock.lock(); }
     void assert_locked(Mutex& mutex) MP_ASSERT_CAPABILITY() MP_ASSERT_CAPABILITY(mutex)