Remove last remnant of nosetests

TestDownload was setting a custom nosetest flag, remove it as we no
longer use nose

test/test_download.py

@@ -65,10 +65,6 @@ def _file_md5(fn):
 
 @is_download_test
 class TestDownload(unittest.TestCase):
-    # Parallel testing in nosetests. See
-    # http://nose.readthedocs.org/en/latest/doc_tests/test_multiprocess/multiprocess.html
-    _multiprocess_shared_ = True
-
     maxDiff = None
 
     COMPLETED_TESTS = {}