Remove unused SpinLock (#19075)

Author: kbenzie

source/ur/ur.hpp

@@ -185,27 +185,6 @@ class ur_lock {
   }
 };
 
-/// SpinLock is a synchronization primitive, that uses atomic variable and
-/// causes thread trying acquire lock wait in loop while repeatedly check if
-/// the lock is available.
-///
-/// One important feature of this implementation is that std::atomic<bool> can
-/// be zero-initialized. This allows SpinLock to have trivial constructor and
-/// destructor, which makes it possible to use it in global context (unlike
-/// std::mutex, that doesn't provide such guarantees).
-class SpinLock {
-public:
-  void lock() {
-    while (MLock.test_and_set(std::memory_order_acquire)) {
-      std::this_thread::yield();
-    }
-  }
-  void unlock() { MLock.clear(std::memory_order_release); }
-
-private:
-  std::atomic_flag MLock = ATOMIC_FLAG_INIT;
-};
-
 // The wrapper for immutable data.
 // The data is initialized only once at first access (via ->) with the
 // initialization function provided in Init. All subsequent access to