tee: Remove unused declarations

Commit 4fb0a5eb ("tee: add OP-TEE driver") declared but never implemented
optee_supp_read()/optee_supp_write().
Commit 967c9cca ("tee: generic TEE subsystem") never implemented tee_shm_init().

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>