sys/dev/disk/dm: Remove dm_dev::dev_type

sys/dev/disk/dm: Remove dm_dev::dev_type

This is unused, and also not necessary.

dm core shouldn't need to be aware of target driver type.
Target dependent actions are handled by target dependent handlers.
dm targets have oop-like structure, so we don't want/need
dm core to be able to do things like below.

Also see d471f1f9 and 49784e7d.

switch (dev->dev_type) {
case DM_LINEAR_DEV:
    do_something_specific_to_linear(); break;
case DM_STRIPE_DEV:
    do_something_specific_to_striped(); break;
case ...:
    ...; break;
}