strip python2-only coding:utf-8 & print_function settings

We're committed to Python 3 at this point, so clean up boilerplate.

Bug: https://crbug.com/gerrit/10418
Change-Id: Ib1719ba2eb65c53b94881a1a1bf203ddfcaaafed
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/292382
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

main.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python3
-# -*- coding:utf-8 -*-
 #
 # Copyright (C) 2008 The Android Open Source Project
 #
@@ -21,7 +20,6 @@ People shouldn't run this directly; instead, they should use the `repo` wrapper
 which takes care of execing this entry point.
 """
 
-from __future__ import print_function
 import getpass
 import netrc
 import optparse