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

pip-check-reqs tool is no longer maintained and has broken with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: Id4217fce07cdf836df9c44dc81246154e5389150

openstack/oslo.concurrency