block: clean up bdrv_drain_all() throttling comments

Since cc0681c4 ("block: Enable the new
throttling code in the block layer.") bdrv_drain_all() no longer spins.
The code used to look as follows:

  do {
      busy = qemu_aio_wait();

      /* FIXME: We do not have timer support here, so this is effectively
       * a busy wait.
       */
      QTAILQ_FOREACH(bs, &bdrv_states, list) {
          while (qemu_co_enter_next(&bs->throttled_reqs)) {
              busy = true;
          }
      }
  } while (busy);

Note that throttle requests are kicked but I/O throttling limits are
still in effect.  The loop spins until the vm_clock time allows the
request to make progress and complete.

The new throttling code introduced bdrv_start_throttled_reqs().  This
function not only kicks throttled requests but also temporarily disables
throttling so requests can run.

The outdated FIXME comment can be removed.  Also drop the busy = true
assignment since we overwrite it immediately afterwards.

Reviewed-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>