ION_DUMMY is an old, deprecated configuration flag in the ION driver
that is now useless. Therefore, remove the useless CONFIG_ION_DUMMY=y.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>