tee: remove tee_shm_va2pa() and tee_shm_pa2va()

We should not need to index into SHMs based on absolute VA/PA.
These functions are not used and this kind of usage should not be
encouraged anyway. Remove these functions.

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