remove_system_on_rhn: do not fail without reason
remove_system_on_rhn: do not fail without reason

with this changes, remove_system_on_rhn will not fail anymore if
there is no instance of the server registred on RHN.

redhat-cip/edeploy,enovance/edeploy,redhat-cip/edeploy,enovance/edeploy,lebauce/edeploy,redhat-cip/edeploy,lebauce/edeploy,redhat-cip/edeploy,lebauce/edeploy,enovance/edeploy,lebauce/edeploy,enovance/edeploy