Stop waiting on ExitedWithError

The state machine is only configured to be able to transition to the
ExitedSuccessfully state from the Stopping state when the ProcessExit
trigger is triggered which means that there is no benefit to doing so.

Author: pgermishuys

src/LittleForker/ProcessSupervisor.cs

@@ -312,12 +312,7 @@ private async Task OnStop(TimeSpan? timeout)
                     // only means the process has _started_ to shut down.
 
                     await CooperativeShutdown.SignalExit(ProcessInfo.Id, _loggerFactory).TimeoutAfter(timeout.Value);
-                    var exited = this.WhenStateIs(State.ExitedSuccessfully);
-                    var exitedWithError = this.WhenStateIs(State.ExitedWithError);
-
-                    await Task
-                        .WhenAny(exited, exitedWithError)
-                        .TimeoutAfter(timeout.Value);
+                    await this.WhenStateIs(State.ExitedSuccessfully).TimeoutAfter(timeout.Value);
                 }
                 catch (TimeoutException)
                 {