Attest: Remove initial attestation get public key API function

It is overkill to implement a dedicated secure function for NS to fetch
initial attestation public key just for test purpose.
Besides, this function to get public key can be confusing as it is not
defined in PSA Initial Attestation API spec.

Remove get public key secure function from NS and S sides to simplify
TF-M initial attestation implementation and interface.

Change-Id: I8d0967698e3d2f2c684194caa9a6234585026a71
Signed-off-by: David Hu <david.hu@arm.com>