Remove unused py33 tox env

It's apparent that Neutron is not expected to gain Py3K support on
its stable/icehouse branch, and so developers are not expected to
run the py33 tox env.

Change-Id: Ieae7819e0489ae19b7fcfb63497e06527c918b5c
Merge "Remove unused py33 tox env" into stable/icehouse