Drop the hardware-self-test that can be triggered from user-space. If
really needed, we can revert it and let it write to a sane area (former
sysfs) and use a get-env to trigger instead.

A user can still run the self-test manually; albeit nuking their
flash due to the write test.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>