Fix confirmations check

[spartucus]

No description provided.

[heifner]

Why do you believe this change is needed?

[spartucus]

> *2 / 3 makes little difference to >= *2/3 + 1, it keeps consistence with others.
For example: https://github.com/EOSIO/eos/blob/master/libraries/chain/block_header_state.cpp#L72-L76

static_assert(std::numeric_limits<uint8_t>::max() >= (config::max_producers * 2 / 3) + 1, "8bit confirmations may not be able to hold all of the needed confirmations");

// This uses the previous block active_schedule because thats the "schedule" that signs and therefore confirms _this_ block
auto num_active_producers = active_schedule.producers.size();
uint32_t required_confs = (uint32_t)(num_active_producers * 2 / 3) + 1;

If you think there's no need or i understand this wrong, no hard feelings to close the PR. :)