tls_credentials: remove TinyCrypt implementation of digest_raw

Moving toward the adoption of PSA crypto APIs, TinyCrypt is going
to be deprecated so we can remove it in favor of newer APIs.

Moreover tls_credentials is mostly meant to store data in the
protected storage meaning that there is a TFM implementation
providing this service. The same TFM can of course be used to
compute sha256 hashing thus saving memory footprint on the NS
side by removing TC.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>