Removes the nano::unchecked_info::confirmed member.

[clemahieu]

This was not serialized to the unchecked table. Additionally, the value was assigned from a comparison of retry counts so the naming of "confirmed" was confusing. Finally, its removal doesn't break any tests so it's not a tested branch of code.

[theohax]

[ RUN      ] active_transactions.fork_replacement_tally
/Users/runner/work/nano-node/nano-node/nano/core_test/active_transactions.cpp:849: Failure
Expected equality of these values:
  max_blocks
    Which is: 10
  election->blocks ().size ()
    Which is: 1

Already on the to-fix list, so CI looks green-ish otherwise