Fixed an exception when destroying a dropdown menu, then rescaling the screen

[Nerogar]

The init function of the CTkScalingBaseClass is called when creating a DropdownMenu. But the destroy function is never called. This can lead to an exception after destroying a CTkOptionMenu (which uses a DropdownMenu internally).

To reproduce:

1. Create a CTkOptionMenu
2. Destroy that CTkOptionMenu
3. Move the window to another screen with a different scaling factor