prover_disk: get rid of useless data

[griddis]

while caching C2 to reduce memory usage

[dunhil]

Bitblade plotter aligns each plot table by fs block_size In result cached C2 eats memory quickly. block_size can be around 1Mb => 260k C2 entries cached, but almost all zeros and usefull

[github-actions[bot]]

'This PR has been flagged as stale due to no activity for over 60 days. It will not be automatically closed, but it has been given a stale-pr label and should be manually reviewed.'

[rostislav]

This has been resolved in PR #337.