logind: drop unused user_tasks_max field

logind: drop unused user_tasks_max field

We would only write to the field, and take the address. All *readers* were
removed in 284149392755f086d0a714071c33aa609e61505e. (The explanation for why
the field wasn't removed back then is that the patch underwent a few iterations,
with the initial version adding translation back and forth. Later versions of
the patch simply emit a warning and ignore the old value. Apparently nobody
noticed that the value became unused.)