openssh: fix warnings about overwritting keys

* they were causing real issues like openssh-sshd-systemd not really RPROVIDED..
  WARNING: Variable key RCONFLICTS_${PN}-sshd (dropbear) replaces original key RCONFLICTS_openssh-sshd ( openssh-sshd-systemd).
  WARNING: Variable key FILES_${PN}-sshd (${sbindir}/sshd ${sysconfdir}/init.d/sshd ${sysconfdir}/ssh/moduli ${sysconfdir}/ssh/sshd_config) replaces original key FILES_openssh-sshd ( ${systemd_unitdir}/system/sshd.socket).
  WARNING: Variable key RPROVIDES_${PN}-sshd (sshd) replaces original key RPROVIDES_openssh-sshd ( openssh-sshd-systemd).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>