[MTD] Remove option for add_mtd_partitions() to not register partitions.

This breaks the dilnetpc map driver, but it could be fixed not to use
that option. We want to simplify the partition handling, and this is a
step towards that.

Remove superfluous 'index' field from private struct mtd_part too, while
we're at it.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>