Fixed 'pip install --user catkin_tools' (#488)

pip passes `--user --prefix=` when the user calls `pip install --user`,
which conflicted with the mutually_exclusive_group being used to parse
these arguments.

The mutually_exclusive_group is not necessary since pip prevents the
user from passing `--user` together with `--prefix` anyhow. By not
checking it here again, we can accept pip passing `--user --prefix=` to
us.

setup.py

@@ -99,9 +99,8 @@ def run(self):
                         'catkin_tools-completion.bash')))
 
 parser = argparse.ArgumentParser(add_help=False)
-prefix_group = parser.add_mutually_exclusive_group()
-prefix_group.add_argument('--user', '--home', action='store_true')
-prefix_group.add_argument('--prefix', default=None)
+parser.add_argument('--user', '--home', action='store_true')
+parser.add_argument('--prefix', default=None)
 
 opts, _ = parser.parse_known_args(sys.argv)
 userbase = site.getuserbase() if opts.user else None