Remove manual generation of UML diagrams; they will be built by Sphinx
itself.
Require to install PlantUML and Graphviz via bindep in
openstack-tox-docs job; ReadTheDocs service already has both PlantUML
and Graphviz installed for documentation builds.
Change-Id: I21ab124a80e7768bc9edd891b975b4e0f8a6e50d
Story: 2004747