refactor: Simplify KeyboardMonitor implementation by removing unnecessary monitor parameter in constructor

Author: lijy91

src/keyboard_monitor_macos.mm

@@ -12,14 +12,13 @@
 
 class KeyboardMonitor::Impl {
  public:
-  Impl(KeyboardMonitor* monitor) : monitor_(monitor) {}
+  Impl() {}
 
   CFMachPortRef eventTap = nullptr;
   CFRunLoopSourceRef runLoopSource = nullptr;
-  KeyboardMonitor* monitor_;
 };
 
-KeyboardMonitor::KeyboardMonitor() : impl_(std::make_unique<Impl>(this)), event_handler_(nullptr) {}
+KeyboardMonitor::KeyboardMonitor() : impl_(), event_handler_(nullptr) {}
 
 KeyboardMonitor::~KeyboardMonitor() {
   Stop();