Removing unneeded from __future__ import statements.

Since we are on Python 3.6 and up, we don't need those anymore.

ebook_converter/data/default_tweaks.py

@@ -1,8 +1,3 @@
-#!/usr/bin/env python2
-# vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai
-# License: GPLv3 Copyright: 2010, Kovid Goyal <kovid at kovidgoyal.net>
-from __future__ import unicode_literals
-
 # Contains various tweaks that affect calibre behavior. Only edit this file if
 # you know what you are doing. If you delete this file, it will be recreated from
 # defaults.