net: openthread: Remove PSA crypto backend workarounds

Remove two workarounds in OpenThread's PSA crypto backend
that were required when Zephyr used pre-1.5 TF-M version:
1. psa_open_key() is no longer needed to reference
   a persistent key
2. psa_cipher_encrypt() can be used to simplify AES
   encryption

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>