arm: arm925t: remove SX1 board

SX1 does not build properly by itself, is not built
as part of MAKEALL arm or MAKEALL -a arm, and is only
present in Makefile, not boards.cfg. As it also has no
entry in MAINTAINERS, it is orphan and non-functional.
Remove it.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>