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pip_missing_reqs tool is no longer maintained and has broken with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: I30bfdeca565a21867ea8c1ddedca5d74293008cd

openstack/oslo.vmware