tools: Remove absolute_import in most tools.

Tweaked by tabbott to not remove it from lister.py, linter_lib, and
friends, since those are intended to support both Python 2 and 3
(we're planning to extract them from the repository).

tools/tests/test_html_branches.py

@@ -1,4 +1,3 @@
-from __future__ import absolute_import
 from __future__ import print_function
 
 import unittest