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/graphics/palette_editor.cc

@@ -2,7 +2,7 @@
 
 #include "absl/status/status.h"
 #include "absl/strings/str_cat.h"
-#include "app/core/performance_monitor.h"
+#include "app/gfx/performance_profiler.h"
 #include "app/gfx/snes_palette.h"
 #include "app/gui/color.h"
 #include "imgui/imgui.h"
@@ -189,7 +189,7 @@ absl::Status DisplayPalette(gfx::SnesPalette& palette, bool loaded) {
 void PaletteEditor::Initialize() {}
 
 absl::Status PaletteEditor::Load() {
-  core::ScopedTimer timer("PaletteEditor::Load");
+  gfx::ScopedTimer timer("PaletteEditor::Load");
   
   if (rom()->is_loaded()) {
     // Initialize the labels