rockchip: Remove partitions env variable for RK356x

rockchip: Remove partitions env variable for RK356x

The partitions env variable is using an outdated partition layout that
is typically expected to be used with older vendor miniloader blobs.

Rockchip devices will run fine using any partition layout if the first
16 MiB of MMC storage is ignored/skipped.

Remove the partitions env variable to stop encourage users a continued
use of this outdated partition layout on RK356x devices.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>