Useless ui.UI class

[jumentous]

Whilst it would appear that the ui.py:UI class used to be intended for something it is only used in 2 places:

1. from start_ui where it is called to choose which UI to launch. However at no point here does it use self so in essence this could/should just be a function.

2. as a base class for deluge/ui/web/web.py:Web, however the init of this class does not chain to the parent class or do anything similar.

I can only imagine that this cross over came from old, removed code and is no longer needed.

This code can simply be merged back into start_ui, if for no other reason that it's less confusing for people reading the code and distinguishing the difference between the _UI and UI classes.

[jumentous]

Note: needs the patch in #1971 to apply cleanly.