power: remove watching for the process of guards

The idea behind was that a guard could be active for a process and when the process dies the guard needs to be removed. This could in theory happen for a guard of an external process that is does not have a fifo. In reality this case is impossible, because any guard that is not created internally, was created via a D-Bus call which in turn will always create a fifo. The fifo itself is sufficient to also catch the death of the process even when it happens during a bolt crash because when bolt re-starts and starts to monitor the fifo again it will receive the corresponding close event. Therefore remove process watching from BoltPower.