sys/dev/disk/dm: Remove dm_table_init_target() call with NULL

These can be removed after 41a68322.

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 h

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;
}

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sys/dev/disk/dm: Remove dm/targets/ directory and move its entries to dm/

Device mapper targets' path
sys/dev/disk/dm/targets/<name>/dm_target_<name>.c
are too deep for no reason.

This commit remov

sys/dev/disk/dm: Remove dm/targets/ directory and move its entries to dm/

Device mapper targets' path
sys/dev/disk/dm/targets/<name>/dm_target_<name>.c
are too deep for no reason.

This commit removes targets/ directory which isn't necessary at all,
assuming userspace has no dependencies on this (and it shouldn't either).

Neither NetBSD (sys/dev/dm/dm_target_*.c) nor Linux kernel (drivers/md/dm*.c)
has a directory tree like this, which is better.

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