Simplify psa_open_key
Simplify psa_open_key

Simplify psa_open_key now that the old method for key creation
(returning a handle to a slot with no key material) no longer exists.

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls