blueprint-execution: Remove step that plumbs firewall rules

When this step was added to blueprint execution, it was required so sled-agent could get firewall rules for newly-deployed Nexus instances. Since then, the ServiceRulePropagator RPW has been added, which plumbs firewall rules periodically.

I think it would be fine to just delete the blueprint execution step entirely, but needs at least some basic testing to ensure that's right. The activation period for ServiceRulePropagator is currently pretty long (5 minutes), so there might be cases for:

• shorten the ServiceRulePropagator period
• have blueprint execution activate ServiceRulePropagator explicitly after deploying zones (I think this would be equivalent to the previous bullet? since we'd now be activating ServiceRulePropagator on the blueprint-execution period)
• have blueprint execution activate ServiceRulePropagator explicitly after deploying zones if any new zones were added (I think this would require more information from sled-agent about what it did to handle a PUT /omicron-zones request)

Start by locating the blueprint-execution step that plumbs firewall rules and reading the ServiceRulePropagator RPW behavior. Check how blueprint execution deploys zones and how sled-agent handles PUT /omicron-zones requests, then add basic tests that establish whether deleting the step preserves firewall-rule propagation and whether an explicit activation or period change is needed.