tee: remove unused tee_shm_pool_alloc_res_mem()

None of the drivers in the TEE subsystem uses
tee_shm_pool_alloc_res_mem() so remove the function.

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