dm: do not set 'discards_supported' in targets that do not need it

The DM target's 'discards_supported' flag is intended to act as an
override.  Meaning, even if the underlying storage doesn't support
discards the DM target will.

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

drivers/md/dm-era-target.c

@@ -1513,7 +1513,6 @@ static int era_ctr(struct dm_target *ti, unsigned argc, char **argv)
 	ti->flush_supported = true;
 
 	ti->num_discard_bios = 1;
-	ti->discards_supported = true;
 	era->callbacks.congested_fn = era_is_congested;
 	dm_table_add_target_callbacks(ti->table, &era->callbacks);