ARM: s3c: remove adc.c

This driver could not be enabled on s3c64xx for a long time
because of the ARCH_MULTIPLATFORM dependency, so remove it.

Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>