Internal NTP: find boundary NTP exlusively via DNS

Release 10 set up the `boundary-ntp.control-plane.oxide.internal` DNS
name. Now that it is out the door, we no longer need to configure each
internal NTP zone with the explicit names of each boundary NTP server.

Closes #6261.

sled-agent/src/services.rs

@@ -2175,11 +2175,6 @@ impl ServiceManager {
                         format!("{BOUNDARY_NTP_DNS_NAME}.{DNS_ZONE}"),
                     );
 
-                for s in ntp_servers {
-                    chrony_config =
-                        chrony_config.add_property("server", "astring", s);
-                }
-
                 let dns_client_service;
                 if dns_servers.is_empty() {
                     dns_client_service = disabled_dns_client_service;

smf/chrony-setup/manifest.xml

@@ -34,7 +34,8 @@
     <propval name="boundary_pool" type="astring" value="" />
     <!--
         Upstream NTP server. May be specifid more than once. (At least one is
-        required.)
+        required for boundary NTP zones; internal NTP zones instead use
+        `boundary_pool` above to find the boundary NTP servers.)
     -->
     <propval name="server" type="astring" value="" />
     <!-- Allowed IPv6 range for clients (typically the rack subnet) -->

zone-setup/src/bin/zone-setup.rs

@@ -475,14 +475,6 @@ maxslewrate 2708.333
             .expect("write to String is infallible");
         }
     } else {
-        // TODO-cleanup: Remove specific boundary NTP servers after R10 is cut;
-        // once all racks are setting up the boundary NTP pool we can drop
-        // individual server lines:
-        // https://github.com/oxidecomputer/omicron/issues/6261
-        for s in servers {
-            writeln!(&mut new_config, "server {s} iburst minpoll 0 maxpoll 4")
-                .expect("write to String is infallible");
-        }
         writeln!(
             &mut new_config,
             "pool {boundary_pool} iburst maxdelay 0.1 maxsources 16",