[Base] Clean up simple message box utility.

src/xenia/base/logging.h

@@ -95,11 +95,6 @@ void AppendLogLine(LogLevel log_level, const char prefix_char,
 // Logs a fatal error and aborts the program.
 void FatalError(const std::string_view str);
 
-// Shows error box
-void ShowErrorMessageBox(std::string m);
-
-// Show info box
-void ShowInfoMessageBox(std::string m);
 }  // namespace xe
 
 #if XE_OPTION_ENABLE_LOGGING