Remove miner optimization

Nobody is using the miner besides testing, there's more efficient CPU miners out there and I don't think bitcoind needs more than a very basic miner for testing and maybe educational purposes.
This PR makes the miner code less efficient but clearer.
Params().AllowMinDifficultyBlocks() is for testnet, yes. It's also called inside UpdateTime and inside GetNextWorkRequired.
But this if is not longer necessary because UpdateTime already updates pblock->nBits in testnet case and there's no hashTarget variable to update with pblock->nBits