Remove some unnecessary ignore comments in vm_service

Change-Id: I9b869a82932bf242c7df3045ca4ca888c2fb3fbd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/274732
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/vm_service/example/vm_service_tester.dart

@@ -41,7 +41,6 @@ void main() {
 
     print('dart process started');
 
-    // ignore: unawaited_futures
     process!.exitCode.then((code) => print('vm exited: ${code}'));
     process!.stdout.transform(utf8.decoder).listen(print);
     process!.stderr.transform(utf8.decoder).listen(print);

pkg/vm_service/test/async_generator_breakpoint_test.dart

@@ -71,7 +71,6 @@ Future testAsync(VmService service, IsolateRef isolateRef) async {
   final hits = <Breakpoint>[];
   await service.streamListen(EventStreams.kDebug);
 
-  // ignore: unawaited_futures
   service
       .evaluate(isolateId, lib.id!, 'testerReady = true')
       .then((Response result) async {

pkg/vm_service/test/common/test_helper.dart

@@ -65,7 +65,6 @@ class _ServiceTesteeRunner {
     }
     if (!pause_on_exit) {
       // Wait around for the process to be killed.
-      // ignore: unawaited_futures
       io.stdin.first.then((_) => io.exit(0));
     }
   }

pkg/vm_service/test/coverage_const_field_async_closure_test.dart

@@ -26,7 +26,6 @@ Future<String> fooAsync(int x) async {
 
 Future<void> testFunction() async {
   await Future.delayed(Duration(milliseconds: 500));
-  // ignore: unawaited_futures
   fooAsync(42).then((_) {});
   debugger();
 }