gitk: always use themed Tk (ttk)

gitk: always use themed Tk (ttk)

gitk added the option to used themed Tk (ttk) in 0cc08ff7dd ("gitk: Add
a user preference to enable/disable use of themed widgets", 2009-09-05).
Using ttk had to be optional as Tk 8.4, then in common use, does not
have ttk. ttk is the default when available, so the ttk code paths are
by now very well tested. gitk also has code paths for the older default
widgets, increasing the maintenance burden. Let's make ttk non-optional
to reduce code complexity in later commits.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>