Define always psa_key_id_t as defined in the PSA
Cryptography API specification independently of
whether the MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER
configuration file is set or not.
As a consequence, get rid of `psa_app_key_id_t` that is
not needed anymore.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>