Calling the org.freedesktop.Tracker.Miner Start() method could trigger
a g_critical() warning if the miner was already started:

    Tracker-CRITICAL **: 10:32:39.592: tracker_miner_start: assertion 'miner->priv->started == FALSE' failed'

Callers of the D-Bus API have no reliable way to know if the miner
is already running, so we should just remove the warning and allow
tracker_miner_start() to be called even when the miner is running.
The same is true of Stop().