rtp: basepay: Don't unset stats on FlushStop

They are still valid and unsetting them here would cause no stats to
ever be updated again until the next state change.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1693>

net/rtp/src/basepay/imp.rs

@@ -1228,8 +1228,6 @@ impl RtpBasePay2 {
                 state.last_pts = None;
 
                 drop(state);
-
-                *self.stats.lock().unwrap() = None;
             }
             _ => (),
         }