dm: Remove max_secure_erase_granularity

The max_secure_erase_granularity boolean of struct dm_target is used in
__process_abnormal_io() but never set by any target. Remove this field
and the dead code using it.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>