remove python warnning from tox

the minimum constraints of psycopg2 is 2.8 now, we can remove the
TODO

Change-Id: I2a37cb38d9dcba22c679a37838b8e5ddd34262c4

Author: wchy1001

tox.ini

@@ -23,8 +23,6 @@ setenv =
   OS_STDERR_CAPTURE=1
   OS_TEST_TIMEOUT=160
   PYTHONDONTWRITEBYTECODE=1
-# TODO(stephenfin): Remove psycopg2 when minimum constraints is bumped to 2.8
-  PYTHONWARNINGS = ignore::UserWarning:psycopg2
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt