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/renumber-migrations

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 
-from __future__ import absolute_import
 from __future__ import print_function
 from six.moves import input, map, range
 import glob