Remove images directory from docs

I can't find a single place these images are used in our docs.
Best I can tell is that middleware docs have a few of them:
http://docs.openstack.org/developer/keystonemiddleware/middlewarearchitecture.html

Change-Id: I6dc36b506452f99488c2a202da5eea518cc51b68
Merge "Remove images directory from docs"