Remove references to CBS -testing repo

Remove references to CBS -testing repo

We no longer frequently update the CentOS Community Build System
-testing repository. Instead, we use the fully-automated devel repos on
ODL's Nexus. Remove docs about it, as want to try to deprecate it in the
next few releases.

Keeping old example repo config files as they may be used by lots of old
code, should likely be preserved long-term.

Change-Id: I2cc60c2ec63a5e9fa9419d63c7ddc7568fa9195b
Jira: INTPAK-182
Signed-off-by: Daniel Farrell <dfarrell@redhat.com>