SeveritySort: Remove undefined column `hosts(_unhandled)_unreachable`

[sukhwinder33445]

Icingadb-web no longer has the unreachable state and the Hoststatesummary columns hosts_unreachable_handled and hosts_unreachable_unhandled are removed. (see: https://github.com/Icinga/icingadb-web/pull/803). The unreachable column is merged as the following:

• hosts_down_handled = (down && (handled || ! reachable)
• hosts_down_unhandled = (down && ! handled && reachable)

fixes https://github.com/Icinga/icingaweb2-module-cube/issues/160