board: sl28: drop unneeded and outdated flash partitions

This board doesn't use the MTD subsystem in u-boot, thus there is no
need to specify the partitions. They are outdated anyway. Just drop
them.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>