Buck: Remove Python 2.6 compatibility workarounds

The check_output method is only available from Python 2.7, so a wrapper
method was introduced for compatibility with 2.6.

The ZipFile class does not include context manager support in Python
2.6, so the file download code was written to explicitly open and close
the zip files.

Since Buck itself now requires at least Python 2.7, these workarounds
are no longer necessary.  Remove them.

Also, tidy up a few minor PEP-8 coding style violations in the modified
files.

Change-Id: I9a1b25ddf7494e25ccadeaed0c1d85cd1ce2fd09

tools/maven/mvn.py

@@ -16,9 +16,8 @@
 from __future__ import print_function
 from optparse import OptionParser
 from os import path, environ
-
+from subprocess import check_output
 from sys import stderr
-from tools.util import check_output
 
 opts = OptionParser()
 opts.add_option('--repository', help='maven repository id')