Fix RPM builds failing on .ssh/known_hosts cleanup

The build-rpm job (at least) is failing on a minor cleanup step at the end of the job.

06:18:41 + rm /home/jenkins/.ssh/known_hosts
06:18:41 rm: cannot remove ‘/home/jenkins/.ssh/known_hosts’: No such file or directory
06:18:41 + echo 'No known_hosts file'
06:18:41 No known_hosts file

https://jenkins.opendaylight.org/releng/view/packaging/job/packaging-build-rpm-fluorine/116/console

I don't know what changed, but either need to remove that step or make it optional or make it only run if file exists.