ION has been deprecated in the upstream Linux kernel since v5.11 [1].
Consequently, it was removed from the linaro-swg/linux branch optee
shortly after the release 3.13.0 of OP-TEE. Remove the corresponding
kernel configuration flags which are now unused.

Link: [1] https://github.com/torvalds/linux/commit/e722a295cf493388dae474745d30e91e1a2ec549


Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>