Safe-guard ssl_conf_remove_psk() for simultaneous raw-opaque PSKs

The code maintains the invariant that raw and opaque PSKs are never
configured simultaneously, so strictly speaking `ssl_conf_remove_psk()`
need not consider clearing the raw PSK if it has already cleared an
opaque one - and previously, it didn't. However, it doesn't come at
any cost to keep this check as a safe-guard to future unforeseen
situations where opaque and raw PSKs _are_ both present.

library/ssl_tls.c

@@ -7426,7 +7426,11 @@ static void ssl_conf_remove_psk( mbedtls_ssl_config *conf )
          * user's responsibility. */
         conf->psk_opaque = 0;
     }
-    else
+    /* This and the following branch should never
+     * be taken simultaenously as we maintain the
+     * invariant that raw and opaque PSKs are never
+     * configured simultaneously. As a safeguard,
+     * though, `else` is omitted here. */
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
     if( conf->psk != NULL )
     {