remove the no longer needed ignore: cancel_subscriptions (#7542)

This has been fixed: https://github.com/dart-lang/linter/issues/317

packages/flutter_tools/lib/src/commands/drive.dart

@@ -84,7 +84,6 @@ class DriveCommand extends RunCommandBase {
   Device get device => _device;
 
   /// Subscription to log messages printed on the device or simulator.
-  // ignore: cancel_subscriptions
   StreamSubscription<String> _deviceLogSubscription;
 
   int get debugPort => int.parse(argResults['debug-port']);