Clean up old weight data

[nazar-pc]

Right now node will store weight data in aux store forever even if pruning is enabled.
While it is very little data, it is still not nice that they are not cleaned.

Implementation might be (or not be) something similar to paritytech/substrate#10748 upstream (BABE)

[stinkymonkeyph]

Hello, I am interested in this issue and willing to give it a shot with guidance. Please let me know how can I help and where to start.

[nazar-pc]

Feel free to give it a try, but as mentioned in another issue I will not have time for proper mentorship, you'll have to find a way around this yourself.