Simplify the encoding of key agreement algorithms

Get rid of "key selection" algorithms (of which there was only one:
raw key selection).

Encode key agreement by combining a raw key agreement with a KDF,
rather than passing the KDF as an argument of a key agreement macro.
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index 7180d73..0be8e51 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -2243,9 +2243,11 @@
  * \param[in,out] generator       The generator object to use. It must
  *                                have been set up with
  *                                psa_key_derivation_setup() with a
- *                                key agreement algorithm
- *                                (\c PSA_ALG_XXX value such that
- *                                #PSA_ALG_IS_KEY_AGREEMENT(\p alg) is true).
+ *                                key agreement and derivation algorithm
+ *                                \c alg (\c PSA_ALG_XXX value such that
+ *                                #PSA_ALG_IS_KEY_AGREEMENT(\p alg) is true
+ *                                and #PSA_ALG_IS_RAW_KEY_AGREEMENT(\p alg)
+ *                                is false).
  *                                The generator must be ready for an
  *                                input of the type given by \p step.
  * \param step                    Which step the input data is for.