mdadm: drop auto= support

mdadm: drop auto= support

According to author (and what was described in man):
"With mdadm 3.0, device creation is normally left up to udev so this is
option is unlikely to be needed"
This was a workaround for kernel 2.6 family issues (partitionable and
non-partitionable arrays hell) and I believe we are far away from it now.

I'm not aware of any usage of it, hence it is removed.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>