Remove obsolete dvipng install
Remove obsolete dvipng install

dvipng is used by Sphinx pngmath extension - but none of the projects
actually uses it as a search for ".. math" reveals. Remove it from
images.

Corresponding system-config change: I87f8109c708011f256df600f5f782c3fbb5f4e52

Change-Id: Id1c421038d9819b52b0018c1c9b117dabddbc69a

noorul/os-project-config,noorul/os-project-config,Tesora/tesora-project-config,openstack-infra/project-config,dongwenjuan/project-config,openstack-infra/project-config,Tesora/tesora-project-config,dongwenjuan/project-config