filesystem: use std for PathExists

Remove custom platform implementation of `PathExists` and replace uses
with `std::filesystem::exists`.

src/xenia/ui/imgui_drawer.cc

@@ -11,7 +11,6 @@
 
 #include "third_party/imgui/imgui.h"
 #include "xenia/base/assert.h"
-#include "xenia/base/filesystem.h"
 #include "xenia/base/logging.h"
 #include "xenia/ui/window.h"
 
@@ -136,7 +135,7 @@ void ImGuiDrawer::SetupFont() {
   // TODO(benvanik): jp font on other platforms?
   // https://github.com/Koruri/kibitaki looks really good, but is 1.5MiB.
   const char* jp_font_path = "C:\\Windows\\Fonts\\msgothic.ttc";
-  if (xe::filesystem::PathExists(jp_font_path)) {
+  if (std::filesystem::exists(jp_font_path)) {
     ImFontConfig jp_font_config;
     jp_font_config.MergeMode = true;
     jp_font_config.OversampleH = jp_font_config.OversampleV = 1;