configs: keystone2: Unwind KERNEL_MTD_PARTS definition

This is more complex than it needs to be and makes converting these
boards over to plain text env files more difficult. Remove setting
mtdparts as the DTS already contain the partitions. While here also
drop the conflicting definitions from the K2 defconfigs.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>