arch/csky: drop architecture

We currently have no internal and no external toolchain for csky.
The website is down (no https:// available, and the http:// index
page is 404).

This commit removes the architecture entry; remnants will be dropped in
followup changes.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Guo Ren <ren_guo@c-sky.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>