mtd: redboot: remove useless code

We do not need to invoke 'mtd_can_have_bb()' before invoking
'mtd_block_isbad()' because the latter already handles the case when the MTD
device does not support bad blocks.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>