dm: remove dummy dm_table definition

This dummy structure definition was required for RCU macros, but it
isn't required anymore, so delete it.

The dummy definition confuses the crash tool, see:
https://www.redhat.com/archives/dm-devel/2017-April/msg00197.html



Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>