Remove busy waiting checks

Remove busy waiting checks

Busy waiting should never happen (at least the shutdown pipe is always
there for the driver to wait for).
When busy waiting happens, i.e. GNUNET_SCHEDULER_run_from_driver is
called without any task ready, it is a programming error (at least I
don't know any valid use case for busy waiting).
Hence, remove the busy waiting checks and let
GNUNET_SCHEDULER_run_from_driver return GNUNET_SYSERR instead in this
case.