doc(statistics.js): remove FIXME for CallStats not working

The problem described by the FIXME has been fixed by CallStats.

modules/statistics/statistics.js

     if(this.callStatsIntegrationEnabled && !this.callStatsStarted) {
         // Here we overwrite the previous instance, but it must be bound to
         // the new PeerConnection
-        // FIXME CallStats does not show the participant after
-        // stopCallStats/startCallStats, the issue is being investigated on both
-        // our and CallStats side, but given how rare this situation should
-        // be, we need to have this change merged. Without it "invalid pcHash"
-        // error is reported(lib calls are made for the old PeerConnection).
         this.callstats = new CallStats(session, settings, this.options);
         Statistics.callsStatsInstances.push(this.callstats);
         this.callStatsStarted = true;