Remove internal function md_process()

It was already marked as internal use only, and no longer used
internally. Also, it won't work when we dispatch to PSA.

Remove it before the MD_LIGHT split to avoid a corner case: it's
technically a hashing function, no HMAC or extra metadata, but we still
don't want it in MD_LIGHT really.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

include/mbedtls/md.h

@@ -471,10 +471,6 @@ int mbedtls_md_hmac(const mbedtls_md_info_t *md_info, const unsigned char *key,
                     const unsigned char *input, size_t ilen,
                     unsigned char *output);
 
-/* Internal use */
-MBEDTLS_CHECK_RETURN_TYPICAL
-int mbedtls_md_process(mbedtls_md_context_t *ctx, const unsigned char *data);
-
 #ifdef __cplusplus
 }
 #endif