refactor(iroh-net): Remove incremental state from reportgen actor (#2180)

## Description

The reportgen actor was carrying this "incremental" state around in
two ways: as an Option in last_report and as a boolean in incremental.
It's simpler to have less state and only carry it as an Option.

## Notes & open questions

## Change checklist

- [x] Self-review.
- ~~[ ] Documentation updates if relevant.~~
- ~~[ ] Tests if relevant.~~

Author: flub

iroh-net/src/netcheck/reportgen.rs

@@ -100,14 +100,12 @@ impl Client {
         let addr = Addr {
             sender: msg_tx.clone(),
         };
-        let incremental = last_report.is_some();
         let mut actor = Actor {
             msg_tx,
             msg_rx,
             netcheck: netcheck.clone(),
             last_report,
             port_mapper,
-            incremental,
             relay_map,
             stun_sock4,
             stun_sock6,
@@ -186,8 +184,6 @@ struct Actor {
     stun_sock6: Option<Arc<UdpSocket>>,
 
     // Internal state.
-    /// Whether we're doing an incremental report.
-    incremental: bool,
     /// The report being built.
     report: Report,
     /// The hairping actor.
@@ -370,7 +366,7 @@ impl Actor {
         let enough_relays = std::cmp::min(self.relay_map.len(), ENOUGH_NODES);
         if self.report.relay_latency.len() == enough_relays {
             let timeout = self.report.relay_latency.max_latency();
-            let timeout = match self.incremental {
+            let timeout = match self.last_report.is_some() {
                 true => timeout,
                 false => timeout * 2,
             };
@@ -470,7 +466,7 @@ impl Actor {
         // If we're doing a full probe, also check for a captive portal. We
         // delay by a bit to wait for UDP STUN to finish, to avoid the probe if
         // it's unnecessary.
-        if !self.incremental {
+        if self.last_report.is_none() {
             // Even if we're doing a non-incremental update, we may want to try our
             // preferred relay for captive portal detection.
             let preferred_relay = self