dm: remove pointless kobject comparison in dm_get_from_kobject

The comparison is always true and the compiler optimizes it out anyway.

Milan offered additional context relative to the original commit
784aae735d ("dm: add name and uuid to sysfs") which introduced the code:
"I think it is just relict of some experiments before I committed this
simple embedded sysfs kobj handling".

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>