fix: Add missing scaling base class destroy in dropdown_menu.

[qwertyzen]

This PR solves the issue where a frame containing DropdownMenu is destroyed and recreated, then the window is scaled.

This PR adds a missing cleanup statement to dropdown_menu.py.

class DropdownMenu(tkinter.Menu, CTkAppearanceModeBaseClass, CTkScalingBaseClass):
    def __init__(self, *args,
                 min_character_width: int = 18,
                 fg_color: Optional[Union[str, Tuple[str, str]]] = None,
                 hover_color: Optional[Union[str, Tuple[str, str]]] = None,
                 text_color: Optional[Union[str, Tuple[str, str]]] = None,
                 font: Optional[Union[tuple, CTkFont]] = None,
                 command: Union[Callable, None] = None,
                 values: Optional[List[str]] = None,
                 **kwargs):
        # call init methods of super classes
        tkinter.Menu.__init__(self, *args, **kwargs)
        CTkAppearanceModeBaseClass.__init__(self)
        CTkScalingBaseClass.__init__(self, scaling_type="widget")

but destroy missing call to CTkScalingBaseClass.destroy(self)

    def destroy(self):
        if isinstance(self._font, CTkFont):
            self._font.remove_size_configure_callback(self._update_font)
        # call destroy methods of super classes
        tkinter.Menu.destroy(self)
        CTkAppearanceModeBaseClass.destroy(self)
        # fix missing