Annotate unused on_done parameter in TaskScheduler::ScheduleOneShotAsync

In TaskScheduler::ScheduleOneShotAsync, the lambda wrapping the user task
receives an on_done callback parameter that is not invoked because the
custom oneshot_on_done completion handler is passed to the user task
instead. Mark the on_done parameter with [[maybe_unused]] to prevent
unused-parameter compiler warnings.

Google-Bug-Id:563046016
PiperOrigin-RevId: 987676467
Change-Id: Iaea1c75e597fc2d5319ef6e571ee5d6d12d5dc33