This also removes the mach/s3c2400 version which was probably never used
due to the fact that we have this line in arch/arm/Makefile:

machine-$(CONFIG_ARCH_S3C2410)          := s3c2410 s3c2400 [...]

This is later used to construct the search path for:

The compiler would be looking into mach-s3c2410 and picking up this
version first.  Any config that was actually expecting the mach-s3c2400
version was therefore producing a broken kernel binary.  Not relying on
any of them anymore would fix that issue.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>