session_time has not been updated during session_length, so relying on it last
activity data will be incorrect, especially if session_length value is high.
Thus rely on regular and properly updated user_lastvisit.
Remove user_lastvisit vs with session_time sync for the same reason.
Also get rid of the session_last_visit field as it floats between
user_lastvisit and session_time and actually is meaningless.
PHPBB3-16470
PHPBB3-14173