Remove stale pip-missing-reqs tox test
Remove stale pip-missing-reqs tox test

pip_missing_reqs tool is no longer maintained and has broken with
release 10 of pip

Refer to:
 http://lists.openstack.org/pipermail/openstack-dev/2018-April/130027.html

Change-Id: I78b50d28d7d3d06f210a7027b2a74426c891b652

openstack/oslo.db,openstack/oslo.db