refactor: remove unnecessary singleton pattern in main.py

Author: FlysonBot

src/mastermind/main/main.py

@@ -15,11 +15,6 @@
 class MainUI:
     """Class to handle the user menu interface."""
 
-    def __new__(cls):
-        if not hasattr(cls, "instance"):
-            cls.instance = super(MainUI, cls).__new__(cls)
-        return cls.instance
-
     def main_menu(self) -> bool:
         """
         Display the main menu and handle user input.