md: remove recovery_disabled

md: remove recovery_disabled

'recovery_disabled' logic is complex and confusing, originally intended to
preserve raid in extreme scenarios. It was used in following cases:
- When sync fails and setting badblocks also fails, kick out non-In_sync
  rdev and block spare rdev from joining to preserve raid [1]
- When last backup is unavailable, prevent repeated add-remove of spares
  triggering recovery [2]

The original issues are now resolved:
- Error handlers in all raid types prevent last rdev from being kicked out
- Disks with failed recovery are marked Faulty and can't re-join

Therefore, remove 'recovery_disabled' as it's no longer needed.

[1] 5389042ffa36 ("md: change managed of recovery_disabled.")
[2] 4044ba58dd15 ("md: don't retry recovery of raid1 that fails due to error on source drive.")

Link: https://lore.kernel.org/linux-raid/20260105110300.1442509-13-linan666@huaweicloud.com
Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>