[infra] Remove unused member from test runner

This was a leftover from a refactoring that was accidentally committed.

Change-Id: Ibbf26cabbfa16d1aba7fe4811977008c46c19c34
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/104002
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: William Hesse <whesse@google.com>

tools/testing/dart/test_runner.dart

@@ -432,7 +432,6 @@ class RunningProcess {
   int pid;
   OutputLog stdout;
   OutputLog stderr = OutputLog();
-  StreamConsumer stdoutConsumer;
   List<String> diagnostics = <String>[];
   bool compilationSkipped = false;
   Completer<CommandOutput> completer;