Bug 1492362: [taskgraph] Don't delete fetches after running a task; r=glandium

In Bug 1466660, we started deleting the fetches after a task had run, to avoid
interference between tasks. It turns out the only tasks this was for were the
`source-test-jsshell` tasks, which were changed to use an absolute directory in
Bug 1465181. However, since Bug 1568460 we've always used a per-task directory
for fetches, so can remove the work-around of removing fethes.

Differential Revision: https://phabricator.services.mozilla.com/D86670

taskcluster/scripts/run-task

@@ -967,11 +967,6 @@ def main(args):
             else:
                 resource_process.terminate()
             resource_process.wait()
-        fetches_dir = os.environ.get('MOZ_FETCHES_DIR')
-        if fetches_dir and os.path.isdir(fetches_dir):
-            print_line(b'fetches', b'removing %s\n' % fetches_dir.encode('utf-8'))
-            shutil.rmtree(fetches_dir)
-            print_line(b'fetches', b'finished\n')
 
 
 if __name__ == '__main__':