Remove call to pk_decrypt() in ssl_server2

We no longer use decrypt TLS 1.2 (never did in 1.3) so we no longer need
this path. Further simplifications could probably be made (we currently
have an enum type with only one possible value...) but for now I'm
trying to keep changes minimal.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index 3c9fb7e..42fa8d6 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -1140,7 +1140,6 @@
 
 typedef enum {
     ASYNC_OP_SIGN,
-    ASYNC_OP_DECRYPT,
 } ssl_async_operation_type_t;
 
 typedef struct {
@@ -1160,7 +1159,6 @@
 static const char *const ssl_async_operation_names[] =
 {
     "sign",
-    "decrypt",
 };
 
 static int ssl_async_start(mbedtls_ssl_context *ssl,
@@ -1261,11 +1259,6 @@
     }
 
     switch (ctx->operation_type) {
-        case ASYNC_OP_DECRYPT:
-            ret = mbedtls_pk_decrypt(key_slot->pk,
-                                     ctx->input, ctx->input_len,
-                                     output, output_len, output_size);
-            break;
         case ASYNC_OP_SIGN:
             ret = mbedtls_pk_sign(key_slot->pk,
                                   ctx->md_alg,