mtd: mtdconcat: return -EOPNOTSUPP if block_markbad is undefined

The main 'mtd_block_markbad()' function returns -EOPNOTSUPP if the
'->block_markbad' method is undefined, and mtdconcat should do the same.
Fix this by simply removing the 'mtd_can_have_bb()' because it is  not
really necessary. It could be treated as an optimization, but this function is
expected to be used so rarely that it does not matter.

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