Fixes #2384.

```
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
```