If a CA is preferred and not the last CA of a project, it should
not be deleted. A user is informed of this with a 409 status code.
Otherwise, the CA can be deleted as well as any record of it in the
CA project list and the CA preferred list.
Change-Id: I9a1ee91252ee17746cfcffd11cba520270d09f21
Closes-bug: #1499876