chore: Fix unnecessary_unwrap clippy

```
warning: called `unwrap` on `self.exit_probe_rtt_at` after checking its variant with `is_none`
   --> quinn-proto/src/congestion/bbr/mod.rs:247:48
    |
238 |             if self.exit_probe_rtt_at.is_none() {
    |             ----------------------------------- help: try: `if let Some(<item>) = self.exit_probe_rtt_at`
...
247 |             } else if is_round_start && now >= self.exit_probe_rtt_at.unwrap() {
    |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_unwrap
    = note: `#[warn(clippy::unnecessary_unwrap)]` on by default
```

Author: larseggert

quinn-proto/src/congestion/bbr/mod.rs

@@ -235,21 +235,25 @@ impl Bbr {
         }
 
         if self.mode == Mode::ProbeRtt {
-            if self.exit_probe_rtt_at.is_none() {
-                // If the window has reached the appropriate size, schedule exiting
-                // ProbeRtt.  The CWND during ProbeRtt is
-                // kMinimumCongestionWindow, but we allow an extra packet since QUIC
-                // checks CWND before sending a packet.
-                if bytes_in_flight < self.get_probe_rtt_cwnd() + self.current_mtu {
-                    const K_PROBE_RTT_TIME: Duration = Duration::from_millis(200);
-                    self.exit_probe_rtt_at = Some(now + K_PROBE_RTT_TIME);
+            match self.exit_probe_rtt_at {
+                None => {
+                    // If the window has reached the appropriate size, schedule exiting
+                    // ProbeRtt.  The CWND during ProbeRtt is
+                    // kMinimumCongestionWindow, but we allow an extra packet since QUIC
+                    // checks CWND before sending a packet.
+                    if bytes_in_flight < self.get_probe_rtt_cwnd() + self.current_mtu {
+                        const K_PROBE_RTT_TIME: Duration = Duration::from_millis(200);
+                        self.exit_probe_rtt_at = Some(now + K_PROBE_RTT_TIME);
+                    }
                 }
-            } else if is_round_start && now >= self.exit_probe_rtt_at.unwrap() {
-                if !self.is_at_full_bandwidth {
-                    self.enter_startup_mode();
-                } else {
-                    self.enter_probe_bandwidth_mode(now);
+                Some(exit_time) if is_round_start && now >= exit_time => {
+                    if !self.is_at_full_bandwidth {
+                        self.enter_startup_mode();
+                    } else {
+                        self.enter_probe_bandwidth_mode(now);
+                    }
                 }
+                Some(_) => {}
             }
         }