InnoDB never supported more than one copy of a redo log.
There were provisions to do that. For Mariabackup, let us clean up
this code.

log_sys_init(): Renamed from log_init().

log_set_capacity(): Renamed from log_calc_max_ages().

log_init(): Renamed from log_group_init(). Remove the parameters
id, space_id. Let the caller invoke log_set_capacity() when needed.

log_group_t: Remove id, space_id, log_groups.

log_t: Replace log_groups with a single log.

recv_find_max_checkpoint(): Declare globally. Remove the first parameter.

xtrabackup_choose_lsn_offset(): Remove (dead code).