r249170 was just plain wrong. The effect of the change is to always
delete a logic volume on status change which is NOT what we want here.

The original code is correct in that when the volume changes status
the driver will only delete the volume if the status is one of the
fatal errors. A drive failure in a mirrored volume is NOT a situtation
where the volume should dissapear.

Reported on freebsd-scsi@:
https://lists.freebsd.org/pipermail/freebsd-scsi/2015-September/006800.html

MFC after: 3 days