Remove unnecessary mark as unused #1098

`ret` is used always at line 1305 in statement:
`if( ( ret = pk_parse_key_pkcs8_unencrypted_der( pk, key, keylen ) ) == 0 )`

library/pkparse.c

@@ -1261,7 +1261,6 @@ int mbedtls_pk_parse_key( mbedtls_pk_context *pk,
         return( ret );
 #endif /* MBEDTLS_PKCS12_C || MBEDTLS_PKCS5_C */
 #else
-    ((void) ret);
     ((void) pwd);
     ((void) pwdlen);
 #endif /* MBEDTLS_PEM_PARSE_C */