firewall: drop useless placeholder
firewall: drop useless placeholder

The PatternFly form guidelines recommend that placeholders are only
required when they make it clear how the input data should be formatted.

martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit