dm: remove unused variable

Since merging the commit identified in Fixes below, we trigger this
compile time warning:

drivers/md/dm.c: In function ‘__map_bio’:
drivers/md/dm.c:1296:24: warning: unused variable ‘md’ [-Wunused-variable]
 1296 |  struct mapped_device *md = io->md;
       |                        ^~

Remove the 'md' variable.

Fixes: 5a6c35f9 ("block: remove direct_make_request")
Signed-off-by: Jens Axboe <axboe@kernel.dk>