Remove PerformanceMonitor and update references to PerformanceProfiler

- Deleted the PerformanceMonitor implementation and header files as functionality has been fully integrated into gfx::PerformanceProfiler.
- Updated all relevant source files to replace PerformanceMonitor and ScopedTimer with their gfx counterparts, ensuring consistent performance monitoring across the application.

src/app/editor/message/message_editor.cc

@@ -6,7 +6,7 @@
 #include "absl/status/status.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
-#include "app/core/performance_monitor.h"
+#include "app/gfx/performance_profiler.h"
 #include "app/core/platform/file_dialog.h"
 #include "app/core/window.h"
 #include "app/gfx/bitmap.h"
@@ -93,7 +93,7 @@ void MessageEditor::Initialize() {
 }
 
 absl::Status MessageEditor::Load() { 
-  core::ScopedTimer timer("MessageEditor::Load");
+  gfx::ScopedTimer timer("MessageEditor::Load");
   return absl::OkStatus(); 
 }