mediatek: drop redmi-ax6000 variant with modified env

mediatek: drop redmi-ax6000 variant with modified env

This variant uses xiaomi factory u-boot and modified u-boot-env &
bootcmd.
By modifying uboot-env, the xiaomi firmware recovery provided in
the vendor u-boot doesn't work anymore. It's possible to put
u-boot into a state where it refuese to take any serial input.
If the u-boot is in this state, users can't restore their
firmware without taking the flash off the board.
We now have a -stock variant where the vendor u-boot is used in
a way that xiaomi firmware recovery still works, and a -ubootmod
variant where we get rid of all xiaomi components, have more
usable space and no uart console lock. These two should cover all
use cases and we don't need this variant anymore.
Drop this redmi-ax6000 variant. Existing users of this variant
should perform a u-boot mod or restore to the -stock layout.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>