miner: Remove 'throttle' feature
The 'throttle' feature was designed to limit resource usage of Tracker. It causes the miner process to spend more time in poll(), as far as I can see, but I'm not convinced that it has a useful effect on the kernel's scheduling of tracker vs other processes.
Hopefully #77 (using systemd to limit resource usage) will be more effective and we can remove the 'throttle' feature.