Remove __future__ imports

All __future__ imports (unicode_literals, print_function) are already
mandatory in py3. Also remove an outdated py2 comment in
presubmit_canned_checks.py

Bug: 1475402
Change-Id: I27cf6a8268f6dd1081f22af782c4c29a975376ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4867135
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

testing_support/coverage_utils.py

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-from __future__ import print_function
-
 import distutils.version
 import os
 import sys

testing_support/fake_repos.py

@@ -5,8 +5,6 @@
 # found in the LICENSE file.
 """Generate fake repositories for testing."""
 
-from __future__ import print_function
-
 import atexit
 import datetime
 import errno

testing_support/test_case_utils.py

@@ -3,8 +3,6 @@
 # found in the LICENSE file.
 """Simplify unit tests based on pymox."""
 
-from __future__ import print_function
-
 import os
 import random
 import string

testing_support/trial_dir.py

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-from __future__ import print_function
-
 import atexit
 import logging
 import os