Upgrade mach to 0.6; stop vendoring mach.

When I originally rewrote Servo's mach bootstrapping (using virtualenv
w/ requirements.txt in #7103), I didn't specify mach as a requirement
because a new version hadn't been published in a while. Now that 0.6
is out, I asked the mach maintainers to publish a new version on PyPI,
so now we can fetch it like the other Python dependencies.

Fixes https://github.com/servo/servo/issues/10728.

python/mach/setup.py

@@ -1,38 +0,0 @@
-# This Source Code Form is subject to the terms of the Mozilla Public
-# License, v. 2.0. If a copy of the MPL was not distributed with this
-# file, You can obtain one at http://mozilla.org/MPL/2.0/.
-
-try:
-    from setuptools import setup
-except:
-    from distutils.core import setup
-
-
-VERSION = '0.3'
-
-README = open('README.rst').read()
-
-setup(
-    name='mach',
-    description='Generic command line command dispatching framework.',
-    long_description=README,
-    license='MPL 2.0',
-    author='Gregory Szorc',
-    author_email='gregory.szorc@gmail.com',
-    url='https://developer.mozilla.org/en-US/docs/Developer_Guide/mach',
-    packages=['mach'],
-    version=VERSION,
-    classifiers=[
-        'Environment :: Console',
-        'Development Status :: 3 - Alpha',
-        'License :: OSI Approved :: Mozilla Public License 2.0 (MPL 2.0)',
-        'Natural Language :: English',
-    ],
-    install_requires=[
-        'blessings',
-        'mozfile',
-        'mozprocess',
-    ],
-    tests_require=['mock'],
-)
-