Remove markers from setup.py.

Basically, it seems like old versions of setuptools are stupid and can't
handle markers. I apparently failed to notice that urllib3 is handling
this by moving all the markers into setup.cfg, meaning that any
setuptools that doesn't look for setup.cfg just automatically gets all
markers for its extras.

This change fixes that up.

setup.py

@@ -59,8 +59,6 @@
               'cryptography>=1.3.4',
               'idna>=2.0.0',
               'certifi',
-          ],
-          'secure:python_version <= "2.7"': [
               "ipaddress",
           ],
           'socks': [