This PR removes a logic in the miner, which was originally intended to help temporary testnets based on ethash from "running off into the future". If the difficulty was low, and a few computers started mining several blocks per second, the ethash rules (which demand 1s delay between blocks) would push the blocktimes further and further away.
The solution was to make the miner sleep while this happened.

Nowadays, this problem is solved instead by PoA chains, and it's recommended to let testnets and devnets be based on clique instead. The existing logic is problematic, since it can cause stalls within the miner making it difficult for remote workers to submit work if the channel is blocked on a sleep.

Credits to Saar Tochner for reporting this via the bug bounty