media: exynos-gsc: remove unused improper CONFIG definition

Defines prefixed with "CONFIG" should be limited to proper Kconfig options,
that are introduced in a Kconfig file.

In the driver code, there is a define for CONFIG_VB2_GSC_DMA_CONTIG, but
this is not used anywhere in the code.

Just remove this unused definition.

No functional change.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>