Remove polling from PVF preparation memory tracker

[mrcnski]

I think a better way to handle this would be to not poll and instead set up a cgroup with an appropriate limit and subscribe to an event once that limit is breached.

Originally posted by @koute in paritytech/polkadot#6565 (comment)

My comment: I'll keep the polling for now, for the purposes of gathering metrics, as we don't yet know what the limit should be. Once we know, we'll be able to implement this suggestion.

Related issue

#718

[mrcnski]

Also, the current issue requires some further research, so I removed the mentor tag here.