Bug 1341214 - Define GECKO in a single location. r=dustin

Instead of every file trying to get the top source directory having an
ad-hoc definition that gets wrong if the files gets moved around for
some reason, define it in a more central location.

taskcluster/taskgraph/taskgraph.py

@@ -4,13 +4,10 @@
 
 from __future__ import absolute_import, print_function, unicode_literals
 
-import os
-
 from .graph import Graph
 from .util.python_path import find_object
 
 TASKCLUSTER_QUEUE_URL = "https://queue.taskcluster.net/v1/task/"
-GECKO = os.path.realpath(os.path.join(__file__, '..', '..', '..'))
 
 
 class TaskGraph(object):