Remove MBEDTLS_KEY_EXCHANGE_RSA_PSK

Remove mentions of MBEDTLS_KEY_EXCHANGE_RSA_PSK that were not guarded by the
configuration option MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED. This finishes the
removal of library code that supports the RSA-PSK key exchange in TLS 1.2.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 5811de5..9e7c52c 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -3670,8 +3670,7 @@
 
 #if defined(MBEDTLS_SSL_ASYNC_PRIVATE) && \
     defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
-    if ((ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA_PSK ||
-         ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA) &&
+    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA &&
         (ssl->handshake->async_in_progress != 0)) {
         /* We've already read a record and there is an asynchronous
          * operation in progress to decrypt it. So skip reading the