Remove the Tk interfaces

These were a constant source of trouble.  Tkinter likely has multiple
memory leaks that OfflineIMAP was tickling.  I never used these, so poof,
goodbye.

offlineimap/ui/__init__.py

@@ -23,13 +23,6 @@ try:
 except ImportError:
     pass
 
-try:
-    import Tkinter
-except ImportError:
-    pass
-else:
-    import Tk
-
 try:
     import curses
 except ImportError: